Add gradient tests for `tf.maximum` and `tf.minimum` (#14077)

* Add gradient tests for `tf.maximum`

Was looking into adding gradient tests for `tf.clip_by_value`
(https://github.com/tensorflow/tensorflow/pull/13998#discussion_r147542917

)
and then noticed that there is no gradient tests in `math_grad_test.py`
for `tf.maximum`. I think it makes sense to add a gradient test to cover
`tf.maximum`.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Also add a gradient test for `tf.minimum`

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>