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_terminal.vim b/src/testdir/test_terminal.vim
index c1ee493..982b464 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -605,9 +605,7 @@
 endfunc
 
 func Test_terminal_servername()
-  if !has('clientserver')
-    return
-  endif
+  CheckFeature clientserver
   call s:test_environment("VIM_SERVERNAME", v:servername)
 endfunc