test: start test runners in Xtest_xdg dir (#37964)

This is a better way to prevent parallel tests from interfering with
each other, as there are many ways files can be created and deleted in
tests, so enforcing different file names is hard.

Using $TMPDIR can also work in most cases, but 'backipskip' etc. have
special defaults for $TMPDIR.

Symlink runtime/, src/, test/ and README.md to Xtest_xdg dir to make
tests more convenient (and symlinking test/ is required for busted).

Also, use README.md instead of test/README.md in the Ex mode inccommand
test, as test/README.md no longer contains 'N' char.

test/functional/vimscript/executable_spec.lua

@@ -20,6 +20,9 @@ describe('executable()', function()
 
   if is_os('win') then
     it('exepath respects shellslash', function()
+      -- test/ cannot be a symlink in this test.
+      n.api.nvim_set_current_dir(t.paths.test_source_path)
+
       command('let $PATH = fnamemodify("./test/functional/fixtures/bin", ":p")')
       eq(
         [[test\functional\fixtures\bin\null.CMD]],
@@ -33,6 +36,9 @@ describe('executable()', function()
     end)
 
     it('stdpath respects shellslash', function()
+      -- Needs to check paths relative to repo root dir.
+      n.api.nvim_set_current_dir(t.paths.test_source_path)
+
       t.matches(
         [[build\Xtest_xdg[%w_]*\share\nvim%-data]],
         call('fnamemodify', call('stdpath', 'data'), ':.')