[analyzer]: Improve test handling with multiple constraint managers

Summary: Modify the test infrastructure to properly handle tests that require z3, and merge together the output of all tests on success. This is required for D28954.

Reviewers: dcoughlin, zaks.anna, NoQ, xazax.hun

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33308

llvm-svn: 305480