TST: fix tests catching unorderable errors for python 3.6
See also https://github.com/MacPython/pandas-wheels/pull/8, seems that the order of int and str is switched in some cases (eg https://travis-ci.org/MacPython/pandas-wheels/jobs/187223139). Therefore making the test here more robust.

Related to https://github.com/pandas-dev/pandas/pull/14684

@jreback not sure if this should be changes in `_is_unorderable_exception` (https://github.com/pandas-dev/pandas/pull/14684/files#diff-779e809bc7bd6d23787580ee9be98729R199) as well. In any case, did not see failing tests related to that in the wheels building for mac. yeah py 3.6. changed the error message, so maybe *should* change ``_is_unorderable_exception`` as well (to make it less strict), not sure though ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15007?src=pr) is 84.78% (diff: 100%)
> Merging [#15007](https://codecov.io/gh/pandas-dev/pandas/pull/15007?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15007   diff @@
==========================================
  Files           145        145          
  Lines         51090      51090          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43315      43316     +1   
+ Misses         7775       7774     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e27b296...da8f8c9](https://codecov.io/gh/pandas-dev/pandas/compare/e27b29697f0dcf9359f01a19edb2f20c6d728b6c...da8f8c984a44d70fdac2ce75d7b4e93c4463a692?src=pr)