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_sha256.vim b/src/testdir/test_sha256.vim
index f6f430b..a1bec97 100644
--- a/src/testdir/test_sha256.vim
+++ b/src/testdir/test_sha256.vim
@@ -20,3 +20,5 @@
   " test for contains non-ascii char:
   call assert_equal('5f78c33274e43fa9de5659265c1d917e25c03722dcb0b8d27db8d5feaa813953', sha256("\xde\xad\xbe\xef"))
 endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab