[CMake] Refactor testing infrastructure

The code for the two OpenMP runtime libraries was very similar.
Move to common CMake file that is included and provides a simple
interface for adding testsuites. Also add a common check-openmp
target that runs all testsuites that have been registered.

Note that this renames all test options to the common OPENMP
namespace, for example OPENMP_TEST_C_COMPILER instead of
LIBOMP_TEST_COMPILER and so on.

Differential Revision: https://reviews.llvm.org/D40082

llvm-svn: 319343