Preserve and reset user roles and capabilities between test runs

[srtfisher]

Continuing from #826...

Mantle is currently taking too aggressive of a stance when cleaning up between test runs. Right now, it resets all meta keys in the tear down for each test run. This can cause issues when attempting to use meta that was registered on init before the test case even started.

We should make it possible to reset the $wp_roles global between test runs. The global should be backed up before any test begins and then restored at the start of each run.

This should be an opt-in feature and not something included with the framework by default.