Use data files for various tests

[CodyCBakerPhD]

Motivation

Swapping to using specific soon-to-be-outdated versions of PyNWB to generate NWB files that trigger certain Best Practices since they can no longer be made in-memory.

A couple other things along the way...

• It's kind of annoying to ask a user to try to install an old version of PyNWB to generate the testing files if they wanted to run the testing suite themselves, so I'll set it up for the files to be hosted on a DANDI staging server where everyone can easily pull from instead of dealing with generating the files locally. This is automatically taken care of by GitHub workflow. It also opens the door to hosting user-provided files from issues, which I have a few of sitting around on some drives.
• With the increased usage of nwbinspector.testing submodule, also enforcing structure of /src/ outer level

[codecov-commenter]

Codecov Report

Merging #288 (a2977cf) into dev (a4a66e7) will increase coverage by 0.01%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##              dev     #288      +/-   ##
==========================================
+ Coverage   93.95%   93.97%   +0.01%     
==========================================
  Files          20       20              
  Lines        1026     1045      +19     
==========================================
+ Hits          964      982      +18     
- Misses         62       63       +1     

Flag	Coverage Δ
unittests	93.97% <90.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/nwbinspector/__init__.py	100.00% <ø> (ø)
src/nwbinspector/checks/behavior.py	100.00% <ø> (ø)
src/nwbinspector/checks/ecephys.py	100.00% <ø> (ø)
src/nwbinspector/checks/general.py	100.00% <ø> (ø)
src/nwbinspector/checks/icephys.py	100.00% <ø> (ø)
src/nwbinspector/checks/images.py	78.94% <ø> (ø)
src/nwbinspector/checks/nwb_containers.py	96.42% <ø> (ø)
src/nwbinspector/checks/nwbfile_metadata.py	98.98% <ø> (ø)
src/nwbinspector/checks/ogen.py	100.00% <ø> (ø)
src/nwbinspector/checks/ophys.py	100.00% <ø> (ø)
... and 10 more