paasio: Pass in object under test
paasio: Pass in object under test

We have tests that test reads for readers and writes for writers, but we
should also test ReadWriteCounters. We can reuse the existing tests by
passing in the object under test.

exercism/xgo,leenipper/xgo,robphoenix/exercism-go,leenipper/xgo,robphoenix/exercism-go,exercism/xgo