Investigate running the test suite without all tests in CI

[arcuru]

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.

[arcuru]

This is fairly dependent on getting #272 fixed first, as otherwise this would double our resource usage on Travis. It would also probably be a good idea to run this only for a single compiler config.

[vaeng]

Superseded