FindGTest: Teach GTEST_ADD_TESTS about TEST_P

Previously the GTEST_ADD_TESTS function would miss parameterized tests
because it only considered TEST and TEST_F.  Add TEST_P to the list of
considered tests and will run all instantiations of this parameterized
test together.

This is perhaps not as correct as searching for all instantiations of
this parameterized test and separating those into separate runs, but
this will at least run tests that were previously missing.

For reference: https://code.google.com/p/googletest/wiki/Documentation
Merge topic 'FindGTest-TEST_P' into next

2e2939c3 FindGTest: Teach GTEST_ADD_TESTS about TEST_P