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.
@Therzok If you think going down this path makes sense (and we actually succeed in making it work on Travis), maybe should we clear the LeaksContainer once we've failed the test in order to avoid polluting (too much) the build log.