test: support running functionaltests in parallel by directory (#37918)

Define a CMake target for every subdirectory of test/functional that
contains functional tests, and a functionaltest_parallel target that
depends on all those targets, allowing multiple test runners to run in
parallel.

On CI, use at most 2 parallel test runners, as using more may increase
system load and make tests unstable.

test/functional/vimscript/executable_spec.lua

@@ -33,9 +33,15 @@ describe('executable()', function()
     end)
 
     it('stdpath respects shellslash', function()
-      eq([[build\Xtest_xdg\share\nvim-data]], call('fnamemodify', call('stdpath', 'data'), ':.'))
+      t.matches(
+        [[build\Xtest_xdg[%w_]*\share\nvim%-data]],
+        call('fnamemodify', call('stdpath', 'data'), ':.')
+      )
       command('set shellslash')
-      eq('build/Xtest_xdg/share/nvim-data', call('fnamemodify', call('stdpath', 'data'), ':.'))
+      t.matches(
+        'build/Xtest_xdg[%w_]*/share/nvim%-data',
+        call('fnamemodify', call('stdpath', 'data'), ':.')
+      )
     end)
   end