test_client.py should be merged and dedup'd into test_async_client.py

[florimondmanca]

This is a piece of technical debt remaining from #544: tests for Client are present (and mostly duplicated) across test_async_client.py and test_client.py.

I've been feeling we should clean that up for a while, and we do need to address it before re-introducing sync, so just tracking it here. :-)

[tomchristie]

I think we’ll likely end up going the other way on this - leave it in because a sync client is imminent (even if the sync dispatch isn’t quite there)

[tomchristie]

Now redundant given #735 (Which reintroduces proper sync tests in test_client.py.)