test controller: Fail test when tox fails

[The-Compiler]

Currently, running tox via the test controller makes the test pass immediately, even if tox is still running, or even failed:

We should attempt to actually get the tox exit status, and show the test as running/passed/failed correctly.

Unfortunately, I'm not quite sure to what extent that's possible with the API we get from VS Code, as long as we still want to run the command in the terminal... Maybe we could use tasks there and listen to onDidEndTaskProcess?

cc @starlord-daniel