Add simple stress test

[gavv]

We should think about adding a stress test which will:

• intensively use multiple senders, receivers, and contexts from parallel goroutines;
• constantly check some invariants, e.g. that no operation hangs and each stream continues to receive more non-zero samples.

It would be also good to run in under race detector.

e2e_test.go from #24 can be used as a base for such a test. See also #101 and https://github.com/roc-streaming/roc-toolkit/issues/298 for some ideas on integration and stress tests.