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/unix.vim b/src/testdir/unix.vim
index b82c63c..366c9b1 100644
--- a/src/testdir/unix.vim
+++ b/src/testdir/unix.vim
@@ -2,7 +2,7 @@
 " Always use "sh", don't use the value of "$SHELL".
 set shell=sh
 
-" Only when the +eval feature is present. 
+" Only when the +eval feature is present.
 if 1
   " While some tests overwrite $HOME to prevent them from polluting user files,
   " we need to remember the original value so that we can tell external systems