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_expr_utf8.vim b/src/testdir/test_expr_utf8.vim
index fad725d..b5937b2 100644
--- a/src/testdir/test_expr_utf8.vim
+++ b/src/testdir/test_expr_utf8.vim
@@ -32,3 +32,5 @@
   call assert_equal('̀', strcharpart('àxb', 1, 1))
   call assert_equal('x', strcharpart('àxb', 2, 1))
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab