[pkg/test] Improve ConcurrentT.Wait()

[RmbRT]

It would be very handy to extend pkg/test/ConcurrentT with a function WaitCtx(ctx, stages...) that fails and aborts the test if the context expires before the requested stages are done. This is important if we have contexts that should limit the duration of tests.

[RmbRT]

#112 implements this a little differently to reduce the complexity of the ConcurrenT's API: instead of having to specify contexts on every blocking call, now, there is one context per ConcurrentT that can be optionally specified in the constructor. All blocking operations will be aborted on context expiry and the test will fail.