Look for ways to speed up end-to-end tests

During CI, we run end-to-end tests against four build flavors: the `nomodule` build and the three experiment builds.

As more long-running tests have been added, it routinely takes ~10 minutes to run a few hundred tests. For contrast, we run more than 12,000 unit tests in a similar amount of time.

This goal of this issue is to explore ways to reduce the running time of our end-to-end tests. Doing so will save time and CPU $$, and reduce overall release turnaround time.

**Link to `main` branch builds:** [here](https://app.circleci.com/pipelines/github/ampproject/amphtml?branch=main)