Unloading and Loading DirectRunner causes crash

I discovered this while experimenting various ways to handle reloading assemblies in the GUI. The engine supports two approaches: (1) Reload and (2) Unload followed by Load.

If you do an Unload and then Load the same assembly, the calls are allowed. In the case of tests being run in a separate process it works. But if the tests are in the same process an exception is thrown because of trying to access an unloaded AppDomain.

Looking at the code, I think it is because the AppDomain is created in LoadPackage and unloaded in UnloadPackage. However, the creation in LoadPackage is conditioned on no package being already loaded.

In principle, the various runners could work either way. That is, they could allow reuse or not allow it at all. But whichever is chosen, the runners should all work the same regardless of whether the tests are being run in or out of process.

For my own project I'm now trying a different approach but I think this should be fixed.

Trace DirectRunner's LoadPackage and UnloadPackage handling of the AppDomain, especially when unloading and loading the same assembly in-process. Compare the behavior with separate-process execution and the other runners. Done means repeated unload/load works consistently, or is consistently rejected, regardless of process mode.