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_substitute.vim b/src/testdir/test_substitute.vim
index 4268aab..7491b61 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -707,7 +707,7 @@
 endfunc
 
 func Test_nocatch_sub_failure_handling()
-  " normal error results in all replacements 
+  " normal error results in all replacements
   func Foo()
     foobar
   endfunc