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)
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 4160605..a3b45a1 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -374,3 +374,5 @@
   " Verify that if a test function ends Vim the test script detects this.
   quit
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab