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.

src/testdir/test_blockedit.vim

@@ -52,7 +52,7 @@ func Test_blockinsert_autoindent()
   let expected =<< trim END
       vim9script
       var d = {
-      	a: (): asdf => 0,
+	a: (): asdf => 0,
       b: (): asdf => 0,
       c: (): asdf => 0,
       }