FIX: Remove test that tests state of warnings before and after assert_warns
TST: Test that the warning registry is empty after assert_warns

Also minor doc change to the statement that this class was
copied from NumPy
TST: Test that assert_warns is reset internally
DOC: Make it explicit that assert_warns clears the warning registry
Merge pull request #3752 from MechCoder/remove_testing_test

[MRG+2] FIX: Modify test that tests state of warnings before and after assert_warns