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)

src/testdir/test_plus_arg_edit.vim

@@ -1,4 +1,5 @@
 " Tests for complicated + argument to :edit command
+
 function Test_edit()
   call writefile(["foo|bar"], "Xfile1")
   call writefile(["foo/bar"], "Xfile2")