Add test assertion methods for checking warnings.
Review Request #10494 — Created April 2, 2019 and submitted
This introduces assertion methods for unit tests that checks if some
code either emits a warning (
assertWarns), or emits no warnings
assertNoWarnings). This simplifies the work that's normally needed
warnings.catch_warnings()) to make basic warning testing
simple. It's mostly a port of Review Board's
Used this in other unit tests, which passed.