Handle leaks

I've recently repaved my box and switched back from the Resharper test runner to Testdriven.net.

And... we're leaking handles. The issue with that is that the dll gets locked and cannot be rewritten over. That would correlate with some reports I've had of such issues from @Allov and @davidebbo.

For instance `UnstageFixture.UnstagingUnknownPathsWithStrictUnmatchedExplicitPathsValidationThrows()` leaks one `DiffSafeHandle`.

I'm currently investigating this further and trying to identify and fix those leaks.

However, I'd be a taker if you had an idea regarding how we could abort the test suite when that happens.
Actually, I'm not getting anything useful when defining LEAKS_IDENTIFYING on Mac. Hmmm, something is stale over here.