[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.
- Ran unit tests.
- Reproduced a known instability in preprocessor indentation which was caught by this new check.
Differential Revision: https://reviews.llvm.org/D42034