patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.

src/testdir/test_syntax.vim

@@ -413,9 +413,8 @@ func Test_highlight_invalid_arg()
 endfunc
 
 func Test_bg_detection()
-  if has('gui_running')
-    return
-  endif
+  CheckNotGui
+
   " auto-detection of &bg, make sure sure it isn't set anywhere before
   " this test
   hi Normal ctermbg=0