chore: run visual and unit tests in separate jobs

Currently, we have a visual CI job that runs yarn test, which runs yarn test:unit && yarn test:visual. This poses a few (admittedly minor) issues:

• Semantically, it feels incorrect to run unit tests as part of a visual job
• yarn test:visual takes a long time to run, it might be more efficient not to run anything else in the same job
• Both yarn test:unit and yarn test:visual produce large output logs, which makes it hard to read the visual job's output

To address these issues, yarn test:unit has been moved to a dedicated unit_tests job, while the visual job will only be responsible for running yarn test:visual.

/cc @ealcantara