ENH: Fix output of assert_frame_equal if indexes differ and check_like=True
- [x] closes #37478 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As described in #37478 This PR fixes some very misleading output when ```assert_frame_equal``` is called with differing index values and ```check_like=True```

I have added a ```check_order``` parameter to the ```assert_index_equal``` function.  This does essentially the same thing as the ```check_like``` parmeter on ```assert_frame_equal``` (but for indexes).  I think ```check_order``` gives a much clearer idea of what the parameter does, but it is a break from the previous naming.  I am unsure whether I should have stuck with ```check_like``` for the new parameter.
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> Thanks @amilbourne for the PR! Looks good
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> I don't know what the pandas policy is on API changes for the testing module (I think it's private but I think it gets used outside our testing suite) so one thing you might need to get agreement on
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> Related to the above if we regard this as a user-facing change you will need to add a whatsnew note

I guess that ``assert_index_equal`` is indeed a part of public API, since the docs are available here https://github.com/pandas-dev/pandas/pull/37479#pullrequestreview-519321722 @ivanovmg, @arw2019  I agree that ```assert_index_equal``` is a part of public API, I just wasn't sure which changes needed a whatsnew note.  I will add one.  Thanks. I made the suggested changes and added a unit test for the new `check_order` param. The build is failing due to a failing test, but it doesn't seem to be anything to do with the changes I made, so I'm assuming that is an anomaly in the build (or a flakey test or something).

The failing test is:
test_slice_irregular_datetime_index_with_nan from pandas/tests/indexing/test_partial.py

This test is also failing for other unrelated PRs, so I don't think it was me :-) @jreback I moved the parameter as asked and rebased.  The build is failing on
`pandas/tests/plotting/test_datetimelike.py::TestTSPlot::test_format_timedelta_ticks_wide`
But I don't think that is related to anything I changed. thanks @amilbourne very nice