Fix check highlight in filecheck. (#42417)
Summary:
* It originally failed to check for cases where highlight token appears more than once.
* Now it repeated tries to find highlight token if one doesn't seem correctly highlighted until end of error message.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42417
Reviewed By: SplitInfinity
Differential Revision: D22889411
Pulled By: gmagogsfm
fbshipit-source-id: 994835db32849f3d7e98ab7f662bd5c6b8a1662e
