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_signals.vim b/src/testdir/test_signals.vim
index c37ea2a..4b6c734 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -133,7 +133,7 @@
   sleep 100m
 
   " We resume after the suspend.  Sleep a bit for the signal to take effect,
-  " also when running under valgrind. 
+  " also when running under valgrind.
   exe 'silent !kill -s CONT ' .. pid_vim
   call WaitForAssert({-> assert_true(filereadable('XautoOut2'))})
   sleep 10m