Add python26 multiline string assertion back.

The assertMultiline equal was removed for assertEqual to be python26
friendly. Testtools has a matcher that provides similar functionality
that is python26 compatible. Use testtools instead of unittest for this
functionality. Remove use of unittest.main().

Change-Id: I348b0ee0192d3164084c20d9356e69f29e1cb86c
Merge "Add python26 multiline string assertion back."