harmonize the testing namespace with tm.TestCase
Currently these two have different places to look for the relevant equality tests: `TestCase` has `assert_numpy_array_equivalent` and `assert_numpy_array_equal` but the testing module has `assert_frame_equal` and `assert_series_equal`.

It would be nice if all substantial code is available form the testing module and `TestCase` implements a small wrapper around these methods. That way someone outside pandas can import `tm` and us ethe methods without subclassing `TestCase`. 
