patch 9.0.1257: code style is not check in test scripts

Problem:    Code style is not check in test scripts.
Solution:   Add basic code style check for test files.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index d3ab99c..8e8be02 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -894,7 +894,7 @@
   let save_term = &term
   let save_ttymouse = &ttymouse
   call test_override('no_query_mouse', 1)
-  
+
   " 'mousetime' must be sufficiently large, or else the test is flaky when
   " using a ssh connection with X forwarding; i.e. ssh -X (issue #7563).
   set mouse=a term=xterm mousetime=600