Support system testing environment

[JessicaSachs]

We should re-use the harness created inside of server/tests/e2e because the project configuration is mostly the same between testingTypes.

• add root level yarn post install so that the system-tests directory has its deps
• support CT-based system tests (vanilla project)
• move existing packages/server/test/e2e into the /system-tests folder in the root
• refactor e2e.it to support finer grained control (mainly so that the system-tests can reuse the project structure (deps + cypress config + folders structure)
• update the how to contribute README
• split tests by timings chore: use circleci timings split for e2e tests #18367
• depcheck to remove unused deps
• do part 2 of chore: Improve CI build time by caching node_modules #16115
• explore adding npm installs/caching node_modules for projects
• fix studio_spec Fix studio_spec system tests #18498

[brian-mann]

• copy only the project you’re working in to .projects
• symlink .projects to /tmp/projects so it doesn’t pollute git AND so it doesn't use node_mods
• research circle ci timings to better understand if we could improve CI performance, and not have to manually balance our spec files
• namespace system-tests as @tooling/system-tests but leave them in the root, but possibly move into a ./tooling/system-tests once thorstens work is merged in (since he’s adding a lot of private packages for our build/script process)
• possibly also could move ./scripts to become ./tooling/scripts if they need their own package.json
• maybe make project take a string and automatically use fixtures
• pass e2e/ct/both if projects can run on both (or just one or the other) as opposed to creating a systemTests.it(...)
• don’t make project use a default, make it required so it’s clear what project is going to be used
• avoid mass file renames to prevent git from cluttering up until we’ve settled on a namespace pattern
• pull and make sure that yarn test 3_user_agent_spec.js runs on your machine, and then merge in develop so you know its all working