Add check-polly-tests build target.

The check-polly-tests target runs regression/unit tests but without checking
formatting. This is useful to not having to reload a file in an open editor
(which eg. clears the undo buffer, moves cursor/window position) when running
polly-update-format.

After this change, the following test targets exist:
 - check-polly-unittests to run unittests only
 - check-polly-tests to run unit and regression tests
 - polly-check-format to check formatting using clang-format
 - check-polly to run them all

As a side-effect, when running check-polly, polly-check-format and run in
parallel (instead of polly-check-format first).

Differential Revision: https://reviews.llvm.org/D24191

llvm-svn: 280654