[FileCheck, unittest] Improve readability of ExpressionFormat

Summary:
Factor out repetetitive code into helper function and split massive
ExpressionFormat method test into separate test for each method,
removing dead code in passing. Also add a MinInt64 and MaxInt64 checks
when testing getMatchingString.

Reviewers: jhenderson, jdenny, probinson, grimar, arichardson

Reviewed By: jhenderson, grimar

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82132