Feature: Speed up tests with more caching

I think we can improve the speed of the tests by tweaking our scripts and workflows. Here are some things I'd like to try:

* Have the builds be distinct jobs that caches the build artifacts that other jobs depend on. Maybe still do a matrix with os and node versions just to be safe?
* test-* jobs should use the cached build results so we are not running prepare-ci so much
* Consider inverting the relationship between dev and prod with e2e tests. Full matrix for prod, and a few runners to sanity check dev. Seems like that should run a little faster? It'll do fewer runtime sanity checks, but I think those are more for logic bugs rather than browser bugs

Start by reviewing the repository's test scripts and CI workflows, especially prepare-ci, test-* jobs, build artifacts, and end-to-end test jobs. Compare current job dependencies and caching across OS and Node-version matrices. Done means the proposed cache and job changes reduce redundant preparation and preserve the intended production and development test coverage.

Written by the indexing model from the issue text.