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_clientserver.vim b/src/testdir/test_clientserver.vim
index db63ce3..ab80bd2 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -30,7 +30,7 @@
 func Test_client_server()
   let cmd = GetVimCommand()
   if cmd == ''
-    return
+    throw 'GetVimCommand() failed'
   endif
   call Check_X11_Connection()