Handle GMs without corresponding check / step name

[beatngu13]

When we try to assign a step name to a GM or vice versa, three situations can occur:

1. We cannot find a GM for the given step name, so we fail the test case. This is the default behavior whenever a test is being executed for the first time.
2. The step name corresponds to a golden master, so we can proceed with the comparison of expected and actual.
3. We have a golden master but without a step name.

AFAIK we currently don't handle no. 3? This can happen if a check is removed or a step renamed. The developer has to decide whether the checkpoint is missing, the GM can be deleted, or something else.

In any case, we should probably track the executed tests and then compare them agains the contents of the recheck folder? Afterwards, we could at least print out a warning when there is a GM but no corresponding has been executed.