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_expand.vim b/src/testdir/test_expand.vim
index c1140aa..d7c393a 100644
--- a/src/testdir/test_expand.vim
+++ b/src/testdir/test_expand.vim
@@ -45,9 +45,9 @@
 
 func Test_expand_tilde_filename()
   split ~
-  call assert_equal('~', expand('%')) 
+  call assert_equal('~', expand('%'))
   call assert_notequal(expand('%:p'), expand('~/'))
-  call assert_match('\~', expand('%:p')) 
+  call assert_match('\~', expand('%:p'))
   bwipe!
 endfunc