Add test for `contiguity_tolerance` kwarg to `iris.plot.pcolormesh`

( and `pcolor`, I guess )
We should really have a test that ensures usage of the kwarg passes down to the appropriate control code.  Either integration or mockist unit test.

Follow-on from https://github.com/SciTools/iris/pull/3162 