TEST: Simplify warning checks

[effigies]

For the 3.0+ series, we can change this idiom:

with clear_and_catch_warnings() as w:
    warnings.simplefilter('always')
    # line that is expected to produce warning
    assert_equal(len(w), 1)
    assert_true(w[0].category is <warningclass>)

To numpy.testing.assert_warns:

with assert_warns(<warningclass>):
    # line that is expected to produce warning

This is generally more readable, IMO, and less prone to disruption when upstream libraries raise extra warnings. For assertions that a line does not raise a warning, we can keep using the old idiom, or write our own.