Minor updates to answer tests infra

[Xarthisius]

PR Summary

While working on pytest port I realized I have some local changes that should be pushed to yt repo first.

1. 3ed51d5 is fairly easy, it's just makes pytest do the same thing as nose. Otherwise some tests were broken on pytest side.
2. 8b35efb is clearly not tested by nose right now...

   >>> import h5py as h5
   >>> h5.File("/tmp/foo.h5", "w").create_dataset(("gas", "density"))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "/home/xarth/codes/xarthisius/yt/venv/lib/python3.8/site-packages/h5py/_hl/group.py", line 156, in 
   create_dataset
       name = self._e(name)
     File "/home/xarth/codes/xarthisius/yt/venv/lib/python3.8/site-packages/h5py/_hl/base.py", line 200, in _e
       name = name.encode('ascii')
   AttributeError: 'tuple' object has no attribute 'encode'
   

[neutrinoceros]

Seems pretty straight forward. Is it meant to be merged ASAP, or would you like to iterate over it ?

[Xarthisius]

Seems pretty straight forward. Is it meant to be merged ASAP, or would you like to iterate over it ?

It can be merged right away. In case I find something new I'll just open more PRs.