[FileCheck] Make Match unittest more flexible

Summary:
FileCheck's Match unittest needs updating whenever some call to
initNextPattern() is inserted before its final block of checks. This
commit change usage of LineNumber inside the Tester object so that the
line number of the current pattern can be queries, thereby making the
Match test more solid.

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

Reviewed By: jhenderson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72913