improve shell parameterization in tests

[nerdvegas]

Iterating over shell in tests currently is not done very well. It is difficult to determine which shells ran and where an error occurs.

Pytest has good support for parameterization, and there's a project that will enable it both for pytest and unittest, which suits rez perfectly (since porting all its tests to pytest will be a decent chunk of work): https://pypi.org/project/parameterized/

Motivation
We would like tests that suck less. This will also help with efforts on #1256

Related Issues/PRs

• add git-bash shell plugin #1256