TST/PERF: Re-write assert_almost_equal() in cython #4398

Add a testing.pyx cython file, and port assert_almost_equal() from
python to cython.  This also fixes a few minor bugs that were in the
python version of assert_almost_equal() and adds more test cases to
test_tests.py

On my machine this brings a modest gain to the suite of "not slow" tests
(160s -> 140s), but on assert_almost_equal() heavy tests, like
test_expressions.py, it shows a large improvement (14s -> 4s).