TST: test 2x2 and 3x3 determinants using explicit formulas

The `test_logdet` and `test_pb_logdet` look kind of weird; the second one doesn't make the same x adjustment so sometimes it tries to take the log of a negative number, and the first function has an additional test.  I think there's a testing feature that lets you `yield` assertions from tests so that later assertions will be checked even if earlier assertions within the same test fail.  But I'm not sure if that's the reason for the two functions.