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
