Implement Refresh and Stop APIs

[JustinGrote]

There are new Refresh button and Stop button APIs. To utilize these, we should:

[ ] Implement the CancellationToken functionality such that any stopped run
[ ] Implement a locking or semaphore mechanism that prevents more than one pester run at a time, and allow pester runs to pre-empt each other. There is some functionality already here at the PowerShell runner level but it needs to also cleanup things at the test runner level.
[ x ] #102 Implement functionality to fully clear and rediscover tests on a refresh. Perhaps have an option to rediscover all tests vs. rediscovering some tests. (Implemented in #153)