Run screenshot test in a separate github actions job

[joekrump]

• Running screenshot tests on the same OS as the reference screenshots were run on can help ensure the screenshot comparisons are more similar. Ex. run screenshot tests on MacOS
• Probably want to still run the rests of the RSpec test suite on an environment more similar to a production one (linux)

Splitting out the screenshot tests can allow this. It would also allow for independent scaling of the runner for the screenshot tests if those tests end up running slow compared with other RSpec tests.

The approach would involve having a new spec directory, something like spec/screenshot_specs that could then be ignored when the rest of the RSpec suite is run. This could be done using the --pattern cmd line arg