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_environ.vim b/src/testdir/test_environ.vim
index b52d102..d9f99f0 100644
--- a/src/testdir/test_environ.vim
+++ b/src/testdir/test_environ.vim
@@ -1,3 +1,5 @@
+" Test for environment variables.
+
 scriptencoding utf-8
 
 func Test_environ()
@@ -42,3 +44,5 @@
   endif
   call assert_equal('', result)
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab