tests: use runtime from build for doc/tags with :help (#10479)

This is better practice in general, and allows to remove the "helptags
ALL" hacks.

Ref: https://github.com/neovim/neovim/issues/8824
Ref: https://github.com/neovim/neovim/commit/f1b67c3453c

* Makefile: fix dependencies with regard to helptags

- use the file as the main target to avoid unnecessary triggering
- use "make oldtest" on Travis to ensure it gets built

test/functional/autocmd/bufenter_spec.lua

@@ -33,9 +33,9 @@ describe('autocmd BufEnter', function()
   end)
 
   it('triggered by ":split normal|:help|:bw"', function()
+    helpers.add_builddir_to_rtp()
     command("split normal")
     command("wincmd j")
-    command("helptags runtime/doc")
     command("help")
     command("wincmd L")
     command("autocmd BufEnter normal let g:bufentered = 1")

test/functional/autocmd/filetype_spec.lua

@@ -8,6 +8,7 @@ describe('autocmd FileType', function()
     before_each(clear)
 
     it("is triggered by :help only once", function()
+        helpers.add_builddir_to_rtp()
         command("let g:foo = 0")
         command("autocmd FileType help let g:foo = g:foo + 1")
         command("help help")

test/functional/ex_cmds/help_spec.lua

@@ -9,6 +9,7 @@ describe(':help', function()
   before_each(clear)
 
   it('window closed makes cursor return to a valid win/buf #9773', function()
+    helpers.add_builddir_to_rtp()
     command('help help')
     eq(1001, funcs.win_getid())
     command('quit')

test/functional/helpers.lua

@@ -800,6 +800,11 @@ function module.parse_context(ctx)
   end, parsed)
 end
 
+function module.add_builddir_to_rtp()
+  -- Add runtime from build dir for doc/tags (used with :help).
+  module.command(string.format([[set rtp+=%s/runtime]], module.test_build_dir))
+end
+
 module = global_helpers.tbl_extend('error', module, global_helpers)
 
 return function(after_each)

test/functional/provider/python3_spec.lua

@@ -97,6 +97,7 @@ describe('python3 provider', function()
   end)
 
   it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    helpers.add_builddir_to_rtp()
     source([=[
       python3 << EOF
       import vim

test/functional/provider/ruby_spec.lua

@@ -101,6 +101,7 @@ end)
 
 describe('ruby provider', function()
   it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    helpers.add_builddir_to_rtp()
     command([=[autocmd BufDelete * ruby VIM::evaluate('expand("<afile>")')]=])
     feed_command('help help')
     eq(2, eval('1+1'))  -- Still alive?