ITK Build/Testing/Coverage doesn't check if modified code is ever exercised in a PR

In #4175 code changes were made on a set of tests which were not part of the default ITK build.

As a result testing never was actually executed on the changes, resulting in the committing of broken code. (The tests were already broken it turned out, but for a different reason before the code change).

The Build/Coverage/Test infrastructure should try to infer whether the code is ever actually used and warn about this.

CC @dzenanz