CTest: Ignore a checkpoint entry for a test that is not pending

CheckResume() reads the checkpoint `ctest -F` resumes from and removes each
test it names.  RemoveTest() erased the iterator std::find returned without
checking it, so a checkpoint naming a test the current run does not have
pending -- one written before the test list changed, say -- erased the end
iterator of a std::list, which is undefined behavior.  Skip such an entry
instead.