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_terminal3.vim b/src/testdir/test_terminal3.vim
index 3343daf..30ff3e7 100644
--- a/src/testdir/test_terminal3.vim
+++ b/src/testdir/test_terminal3.vim
@@ -57,6 +57,8 @@
     term ++shell dir.com /b runtest.vim
     call WaitForAssert({-> assert_match('runtest.vim', term_getline(bufnr(), 1))})
     bwipe!
+  else
+    throw 'Skipped: does not work on this platform'
   endif
 endfunc