[ClangTidy] Separate tests for infrastructure and checkers

Summary:
This change moves tests for checkers and infrastructure into separate
directories, making it easier to find infrastructure tests. Tests for
checkers are already easy to find because they are named after the
checker. Tests for infrastructure were difficult to find because they
were outnumbered by tests for checkers. Now they are in a separate
directory.

Reviewers: jfb, jdoerfert, lebedev.ri

Subscribers: srhines, nemanjai, aheejin, kbarton, christof, mgrang, arphaman, jfb, lebedev.ri, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68807

llvm-svn: 374540