test: fix s390x failures

Problem:
failures in s390x CI.

Solution:
- runtime/lua/man.lua: parse_path() can return nil but 3 callers didn't handle it.
- skip some tests on s390x.

TODO:

- TODO: why "build/bin/xxd is not executable" on s390x?
- TODO: other failures, not addressed (see below).

OTHER FAILURES:

    FAILED   test/functional/treesitter/fold_spec.lua @ 87: treesitter foldexpr recomputes fold levels after lines are added/removed
    test/functional/treesitter/fold_spec.lua:95: Expected objects to be the same.
    Passed in:
    (table: 0x4013c18940) {
      [1] = '0'
      [2] = '0'
      [3] = '0'
     *[4] = '0'
      [5] = '0'
      ...
    Expected:
    (table: 0x4005acf900) {
      [1] = '0'
      [2] = '0'
      [3] = '>1'
     *[4] = '1'
      [5] = '1'
      ...

    stack traceback:
            (tail call): ?
            test/functional/treesitter/fold_spec.lua:95: in function <test/functional/treesitter/fold_spec.lua:87>

    FAILED   test/functional/treesitter/select_spec.lua @ 52: treesitter incremental-selection works
    test/functional/treesitter/select_spec.lua:63: Expected objects to be the same.
    Passed in:
    (string) 'bar(2)'
    Expected:
    (string) 'foo(1)'

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:63: in function <test/functional/treesitter/select_spec.lua:52>

    FAILED   test/functional/treesitter/select_spec.lua @ 69: treesitter incremental-selection repeat
    test/functional/treesitter/select_spec.lua:82: Expected objects to be the same.
    Passed in:
    (string) '2'
    Expected:
    (string) '4'

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:82: in function <test/functional/treesitter/select_spec.lua:69>

    FAILED   test/functional/treesitter/select_spec.lua @ 98: treesitter incremental-selection history
    test/functional/treesitter/select_spec.lua:111: Expected objects to be the same.
    Passed in:
    (string) 'bar(2)'
    Expected:
    (string) 'foo(1)'

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:111: in function <test/functional/treesitter/select_spec.lua:98>

    FAILED   test/functional/treesitter/select_spec.lua @ 186: treesitter incremental-selection with injections works
    test/functional/treesitter/select_spec.lua:201: Expected objects to be the same.
    Passed in:
    (string) 'lua'
    Expected:
    (string) 'foo'

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:201: in function <test/functional/treesitter/select_spec.lua:186>

    FAILED   test/functional/treesitter/select_spec.lua @ 216: treesitter incremental-selection with injections ignores overlapping nodes
    test/functional/treesitter/select_spec.lua:231: Expected objects to be the same.
    Passed in:
    (string) ' )'
    Expected:
    (string) ' foo('

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:231: in function <test/functional/treesitter/select_spec.lua:216>

    FAILED   test/functional/treesitter/select_spec.lua @ 307: treesitter incremental-selection with injections handles disjointed trees
    test/functional/treesitter/select_spec.lua:337: Expected objects to be the same.
    Passed in:
    (string) 'int'
    Expected:
    (string) '1}'

    stack traceback:
            (tail call): ?
            test/functional/treesitter/select_spec.lua:337: in function <test/functional/treesitter/select_spec.lua:307>

    ERROR    test/functional/treesitter/parser_spec.lua @ 562: treesitter parser API can run async parses with string parsers
    test/functional/treesitter/parser_spec.lua:565: attempt to index a nil value

    stack traceback:
            test/functional/testnvim/exec_lua.lua:124: in function <test/functional/testnvim/exec_lua.lua:105>
            (tail call): ?
            (tail call): ?
            test/functional/treesitter/parser_spec.lua:563: in function <test/functional/treesitter/parser_spec.lua:562>

    FAILED   test/functional/core/job_spec.lua @ 1157: jobs jobstop() kills entire process tree #6530
    test/functional/core/job_spec.lua:1244: retry() attempts: 94
    test/functional/core/job_spec.lua:1246: Expected objects to be the same.
    Passed in:
    (table: 0x401dd74b30) {
      [name] = 'sleep <defunct>'
      [pid] = 33579
      [ppid] = 1 }
    Expected:
    (userdata) 'vim.NIL'

    stack traceback:
            test/testutil.lua:89: in function 'retry'
            test/functional/core/job_spec.lua:1244: in function <test/functional/core/job_spec.lua:1157>

runtime/lua/man.lua

@@ -471,6 +471,9 @@ local function format_candidate(path, psect)
     return ''
   end
   local name, sect = parse_path(path)
+  if not name or not sect then
+    return ''
+  end
   if sect == psect then
     return name
   elseif sect:match(psect .. '.+$') then -- invalid extensions
@@ -642,11 +645,13 @@ function M.goto_tag(pattern, _, _)
 
   for _, path in ipairs(paths) do
     local pname, psect = parse_path(path)
-    ret[#ret + 1] = {
-      name = pname,
-      filename = ('man://%s(%s)'):format(pname, psect),
-      cmd = '1',
-    }
+    if pname and psect then
+      ret[#ret + 1] = {
+        name = pname,
+        filename = ('man://%s(%s)'):format(pname, psect),
+        cmd = '1',
+      }
+    end
   end
 
   return ret
@@ -745,6 +750,9 @@ function M.open_page(count, smods, args)
   end
 
   name, sect = parse_path(path)
+  if not name or not sect then
+    return 'no manual entry for ' .. (name or path)
+  end
   local buf = api.nvim_get_current_buf()
   local save_tfu = vim.bo[buf].tagfunc
   vim.bo[buf].tagfunc = "v:lua.require'man'.goto_tag"