Add initial tests for update_{llc_,cc_,}test_checks.py

Summary:
This commit adds basic tests for these update script to validate that
they still work as expected. In the future we could extend these tests
whenever new features are added to avoid introducing regressions.

Reviewers: xbolva00, MaskRay, jdoerfert

Reviewed By: jdoerfert

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70660