[lit] Add more testing instructions to README

Summary:
r283710 introduced two regressions, one to llvm-lit, and the other to
lit executables that were installed via setuptools. Add instructions on
how to test for these regressions in the future.

Reviewers: ddunbar, delcypher, beanz, chapuni, cmatthews, echristo

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25459

llvm-svn: 284919