Test other histograms in 'test_root.py'

[LovelyBuggies]

https://github.com/scikit-hep/aghast/blob/3dcafdd1cb9ff5f903570ccd587e65116c12418c/python/tests/test_root.py#L118-L144

'test_root.py' only checks 1-d ROOT histogram classes. We should add [TH2*] and [TH3*] options. Besides, we can create some demo histograms to test, just like what boost_histogram has done.

I'll do it if time permits.