Ignore expected warnings in tests

[adrianlipow]

Since #63 added the warnings module, running tests now displays a summary of all warnings. We could suppress those via the --disable-warnings flag, but it probably makes sense not to have to remember that every time we run tests (and this suppresses all warnings, even those we might not be expecting and want to see).

We have a set of tests where we expect warnings and test for their existence, so we can tell pytest to ignore warnings on those tests. This cleans up the output of running tests.