patch 8.2.1432: various inconsistencies in test files

Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)

src/testdir/test_gui_init.vim

@@ -58,3 +58,5 @@ func Test_set_guioptions_for_p()
     throw skipped
   endif
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab