testsuite: allow accepting of fine grained results [skip ci]

Summary:
Sometimes we need to be able to mass accept changes that are platform
specific and that can't be normalized away using our string formatters.

e.g. differences in I/O manager errors or behaviors. This allows one
to do so easier than before and less error prone.

I have updated the docs and made it clear this should only be used
when a normalizer won't work:

https://ghc.haskell.org/trac/ghc/wiki/Building/RunningTests/Updating

Test Plan: Manually tested while working on new I/O manager

Reviewers: bgamari

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4549

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