[llvm-exegesis] Add lit tests.

Summary:
Right now we only have unit tests. This will allow testing the whole
tool. Even though We can't really check actual values, this will avoid
regressions such as PR39055.

Reviewers: gchatelet, alexshap

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D52407

llvm-svn: 342953