Patch unit tests by updating OTIO_PLUGIN_MANIFEST_PATH env var

[douglascomet]

The unit tests modules were not functional without registering otio_svg_adapter/plugin_manifest.json with the OTIO_PLUGIN_MANIFEST_PATH env var.

[apetrynet]

@douglascomet Did the unit tests fail locally after installing the plugin in your virtual environment?
From memory:

python3 -m venv .venv
source .venv/bin/activate

pip install --upgrade pip pytest
pip install opentimelineio==0.17.0.dev1

# installs the plugin with symlinks
pip install -e .

[douglascomet]

@douglascomet Did the unit tests fail locally after installing the plugin in your virtual environment? From memory:

python3 -m venv .venv
source .venv/bin/activate

pip install --upgrade pip pytest
pip install opentimelineio==0.17.0.dev1

# installs the plugin with symlinks
pip install -e .

@apetrynet, I did not think to try that! The unit-tests do in fact work and succeed if the plugin is installed into the virtual environment like that.

I remember those being the steps for developing and testing with the main OTIO repo but those steps are not currently mentioned in the README.md of this repo.

[apetrynet]

Glad to hear it works.
Good point about not mentioning this in the README. I'll add that to my notes on prepping the adapters for the openntimelineio 0.17.0 release.

How do you feel about closing this PR?
My thoughts are that we should rely on the plugin system for checking that the adapter installs correctly and passes unit tests.
Any input @reinecke?

[douglascomet]

@apetrynet, I don't mind closing this PR now knowing that the adapter repos should be dev-ed the same way as the main OTIO repo. I didn't know the plugin system would auto-magically find the plugins just by being in the same environment.

We can also close OpenTimelineIO/otio-cmx3600-adapter#7, which I made for the cmx_3600 adapter because it also didn't have any directions on how it should be dev-ed in its README.md.

Also, fwiw, the svg adapter repo does not seem to have any CI currently while the cmx_3600 adapter repo does.

[apetrynet]

Let's see what other think before we land on closing or not.
When it comes to the CI it looks like there's a setting that requires approval for it to run.