[FileCheck] Clean and improve unit tests

Summary:
Clean redundant unit test checks (codepath already tested elsewhere) and
add a few missing checks for existing numeric substitution and match
logic.

Reviewers: jhenderson, jdenny, probinson, grimar, arichardson, rnk

Reviewed By: jhenderson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72912