[clang-format] In tests, expected code should be format-stable

Summary: Extend various verifyFormat helper functions to check that the
expected text is "stable". This provides some protection against bugs
where formatting results are ocilating between two forms, or continually
change in some other way.

Testing Done:

* Ran unit tests.

* Reproduced a known instability in preprocessor indentation which was
  caught by this new check.

Reviewers: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42034

llvm-svn: 329231