Check style on test_expectations.txt files before commit

Check style on test_expectations.txt files before commit
https://bugs.webkit.org/show_bug.cgi?id=76484

Reviewed by Adam Barth.

Unlike other style rules, if you get the syntax of the test_expectations.txt
file wrong, the layout tests won't run. Also, this check is simple and only
slows down committing if you actually modify one of the test_expectations.txt files.

* Scripts/webkitpy/tool/steps/commit.py:
(Commit._check_test_expectations):
(Commit.run):
* Scripts/webkitpy/tool/steps/commit_unittest.py: Added.
(CommitTest):
(CommitTest.test_check_test_expectations):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105213 268f45cc-cd09-0410-ab3c-d52691b4dbfc