Revert "make fails when a unit test fails."

I have a better fix for this...

This reverts commit d5682a891a0a9444c3f646cc6ab1cbf40905e6e2.
Make now returns the exit code of any failing test

Run all unit tests via a script that saves the exit code of the last
failing test and returns it at the end. This way all unit tests are
run but Make will return non-zero if one of them fails.