[UpdateTestChecks] Share the code to parse RUN: lines between all scripts

Summary:
This commit also introduces a common.debug() function to avoid many
`if args.verbose:` statements. Depends on D70428.

Reviewers: xbolva00, MaskRay, jdoerfert

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70432