test/vim.validate(): assert normalized stacktrace

- The previous commit lost information in the tests. Instead, add some
  more "normalization" substitutions in pcall_err(), so that the general
  shape of the stacktrace is included in the asserted text.
- Eliminate contains(), it is redundant with matches()

test/functional/lua/commands_spec.lua

@@ -43,7 +43,7 @@ describe(':lua command', function()
     eq({'', 'ETTS', 'TTSE', 'STTE'}, curbufmeths.get_lines(0, 100, false))
   end)
   it('throws catchable errors', function()
-    eq([[Vim(lua):E5107: Error loading lua [string ":lua"]:1: unexpected symbol near ')']],
+    eq([[Vim(lua):E5107: Error loading lua [string ":lua"]:0: unexpected symbol near ')']],
        pcall_err(command, 'lua ()'))
     eq([[Vim(lua):E5108: Error executing lua [string ":lua"]:1: TEST]],
        exc_exec('lua error("TEST")'))