Add additional NWB export pytests

[kabilar]

See datajoint/workflow-array-ephys#18

[CBroz1]

Duplicate issue datajoint/workflow-array-ephys#52 content:

• The current pytests use subject5 which is SpikeGLX data.
• The current pytests are comprehensive and there are minimal differences between SpikeGLX and Open Ephys data once in the DataJoint Elements tables, but it would be good to also add pytests for Open Ephys data for when loading from source files.