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_arglist.vim b/src/testdir/test_arglist.vim
index 198ca98..820d8b9 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -242,11 +242,11 @@
 
 func Test_args_with_quote()
   " Only on Unix can a file name include a double quote.
-  if has('unix')
-    args \"foobar
-    call assert_equal('"foobar', argv(0))
-    %argdelete
-  endif
+  CheckUnix
+
+  args \"foobar
+  call assert_equal('"foobar', argv(0))
+  %argdelete
 endfunc
 
 " Test for 0argadd and 0argedit