feat(test): use nvim_exec in helpers.source() #16064

helpers.source() was a hack to work around the lack of anonymous
:source. Its "create tempfile" behavior is not a required part of most
tests that use it.

Some tests still need the old "create tempfile" behavior either because
they test SID behavior, or because of missing nvim_exec features: #16071

test/functional/vimscript/lang_spec.lua

@@ -11,6 +11,7 @@ describe('vimscript', function()
       return
     end
     source([[
+      let s:foo = 1
       func! <sid>_dummy_function()
         echo 1
       endfunc