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_exec_while_if.vim

@@ -51,3 +51,5 @@ function Test_exec_while_if()
         \ "8",
         \ "9x"], getline(1, 10))
 endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab