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_dllpath.vim b/src/testdir/test_expand_dllpath.vim
index 76bd787..8e48972 100644
--- a/src/testdir/test_expand_dllpath.vim
+++ b/src/testdir/test_expand_dllpath.vim
@@ -4,12 +4,12 @@
   let $TEST_EXPAND_DLLPATH = '/dllpath/lib' . substitute(a:optname, '\zedll$', '.', '')
   execute 'let dllpath_save = &' . a:optname
   try
-    execute 'set ' . a:optname . '=$TEST_EXPAND_DLLPATH' 
-    execute 'call assert_equal("' . $TEST_EXPAND_DLLPATH . '", &' . a:optname . ')' 
+    execute 'set ' . a:optname . '=$TEST_EXPAND_DLLPATH'
+    execute 'call assert_equal("' . $TEST_EXPAND_DLLPATH . '", &' . a:optname . ')'
 
     execute 'set ' . a:optname . '=~' . $TEST_EXPAND_DLLPATH
     let home = substitute($HOME, '\\', '/', 'g')
-    execute 'call assert_equal("' . home . $TEST_EXPAND_DLLPATH . '", &' . a:optname . ')' 
+    execute 'call assert_equal("' . home . $TEST_EXPAND_DLLPATH . '", &' . a:optname . ')'
   finally
     execute 'let &' . a:optname . ' = dllpath_save'
     let $TEST_EXPAND_DLLPATH = ''