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_signs.vim b/src/testdir/test_signs.vim
index 76cefa2..95e8d44 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -1775,10 +1775,10 @@
   let lines =<< trim END
 	call setline(1, [repeat('x', 75), 'mmmm', 'yyyy'])
 	call cursor(2,1)
-   	sign define s1 texthl=Search text==>
-   	sign define s2 linehl=Pmenu
+	sign define s1 texthl=Search text==>
+	sign define s2 linehl=Pmenu
 	redraw
-   	sign place 10 line=2 name=s1
+	sign place 10 line=2 name=s1
   END
   call writefile(lines, 'XtestSigncolumn', 'D')
   let buf = RunVimInTerminal('-S XtestSigncolumn', {'rows': 6})