Support running TestingServices in parallel

Tests that start multiple servers using `@BeforeSuite` in multiple testing services may be slower than they need to be as these `@BeforeSuite` methods are run sequentially.

It might be nice to support running these methods in parallel. I think if we do this it should probably be the only mode of operation to avoid adding too many options to the API. This may require updates to existing clients to add missing `@DependsOn` annotations.

Alternatively we could support an async version of `@BeforeSuite` etc that perhaps returns a future which we wait on before starting the tests. That way we don't have an on/off control in the API but clients don't start getting their code unexpectedly running in parallel automatically.