Investigate running the test suite without all tests in CI

This was brought up in #234.

It would seem that at least the one test suite wouldn't compile without defining `EXERCISM_RUN_ALL_TESTS`, as the test suite used a global variable that was unused on the first test and everything else was `#ifdef`'d out. We should consider modifying the root CMakeLists file to compile a new test target for every test without that defined just to make sure we still pass the tests. It's a bad experience if they can't use just the initial test in the test suite.