Cleanup Manticore workspace when a test passes

It's nice if testing doesn't produce unnecessary artifacts after running with success but keeps relevant info on fail for further inspection.

Pytest should have a nice way of creating fixtures that can post-process a test based on whether it failed or not https://docs.pytest.org/en/latest/example/simple.html#making-test-result-information-available-in-fixtures

If a test fails, then keep the Manticore workspace, and, if possible, print the associated failed test's workspace directory in the pytest output to know where the user should look.

If a test passes, then remove the Manticore workspace.

Since some of our tests are structured differently, the exact implementation will likely look different for different tests.

@reaperhulk Do you have any comments, suggestions, or references for doing something like this?