Extract model-like TestExpectationLine and TestExpectationFile from TestExpectations.

Extract model-like TestExpectationLine and TestExpectationFile from TestExpectations.
https://bugs.webkit.org/show_bug.cgi?id=64386

This is the first step in converting TestExpectations to a real model.
* TestExpectationsLine represents a line in the test_expectations.txt file, and
* TestExpectationsFile represents the file, which is a collection of lines.

Reviewed by Adam Barth.

* Scripts/webkitpy/layout_tests/models/test_expectations.py:
* Scripts/webkitpy/layout_tests/models/test_expectations_unittest.py:
* Scripts/webkitpy/style/checkers/test_expectations_unittest.py:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@90920 268f45cc-cd09-0410-ab3c-d52691b4dbfc