TST: Deprecate assert_numpy_array_equivalent
Closes #10427.

add this to the deprecation issue for removal in 0.18
 Sure. Also added `testing.assert_isinstance` #10458 to the list.
 Is there a reason we don't use `np.testing.assert_array_equal` (but our own `array_equivalent` implementation)?
 `array_equivalent` is much more general as it handles all nulls for any dtypes (while numpy is restricted to floats). I may do nat's but I don't think so.
 @sinhrks Can you then add a note in the docstring of `assert_numpy_array_equal` that it is similar to `np.testing.assert_array_equal` but more flexible regarding NaNs (so it is clear why this function exists/is used)
 OK, added docstring. Also I found some test case uses `numpy.testing.assert_array_equal` imported to `pandas.util.testing` namespace. Fixed them and deprecate `assert_array_equal` also.
 Ah,  `np.testing.assert_array_equal` behaves slightly different from current `assert_numpy_array_equal`. I'm going to fix test cases to pass `assert_numpy_array_equal` which is more strict. Lmk if I should leave `np.testing.assert_array_equal`.

```
np.testing.assert_array_equal(np.array([1, 1]), 1)
# no error 

pd.util.testing.assert_numpy_array_equal(np.array([1, 1]), 1)
# AssertionError: [1 1] is not equivalent to 1.
```
 Found some cases are affected by  #10546. Revisit later.
 Applied a fix done in #10546. I think this is ready for review.
 @sinhrks looks good to me! (just needs a rebase)

Do we have a general rule for ourselves whether to use `tm.assert_...` or `self.assert_...`? They are available on both, but I saw you now changed some from self to tm.
 the usual issue are we promoting using `self.assert_numpy_array_equal` or `tm.assert_numpy_array_equal`. prob don't care so as long as both work.

pls rebase right before you merge as this touches lots of code.
 @jorisvandenbossche There is no policy, sometimes `self`, `tm.` or directly import each funcs. 

Merging this.
 Super!
 very nice

now all we need is some kind of checker script that we can run in Travis to flag things 
kind of like a linter

eg might be as simple as greping for
np.testing and such
