Test test::join usage in tests

[Thomasdezeeuw]

Following #476 I would like additional tests to make sure when we call test::join (join_many/join_all) in tests it actually ensures the actors ran successfully as various tests depends on this. Otherwise test failures are not reported.

[Thomasdezeeuw]

I've just found out that panics in the spawn actors using test::try_spawn_local (and related) don't actually cause a test to fail. That seems like a problem.

[Thomasdezeeuw]

Maybe #476 can help here.