Update PyNWBIOTest.m

[ehennestad]

Added pythonPath as a variable that can be loaded from env.mat. pythonDir option does not work if the python executable is named python3

Motivation

Can not run python-related tests if the python executable is named python3

How to test the behavior?

N/A

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[ehennestad]

@lawrence-mbf