Exercise generator doesn't skip excluded tests

[BNAndras]

In #206, I noticed the generated tests added tests marked as not included. Fixing this might get a little complicated. The tests are generated directly from the canonical data in the problem-specifications repo. In there, the tests being replaced by another aren't marked, but the tests that do replace another include the replaced test's GUID. So the test generator should track test case GUIDs. Then if one shows up in a reimplements key, the generator can delete the original test case, leaving only the test case that reimplements it.