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_blob.vim b/src/testdir/test_blob.vim
index 4a957aa..cccecb7 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -498,7 +498,7 @@
       call assert_equal(b[-3 :], br5)
       VAR br6 = readblob('Xblob', -3, 2)
       call assert_equal(b[-3 : -2], br6)
-      
+
       #" reading past end of file, empty result
       VAR br1e = readblob('Xblob', 10000)
       call assert_equal(0z, br1e)