Gate new and updated tests based on performance and flakiness

### Summary

Check newly created and updated tests meet performance and flakiness thresholds and enable developers to meet these standards.
I wrote a this design doc on [Keeping AMP Tests Fast](https://docs.google.com/document/d/1o9AaJ8-eKMRxcrLrqLQwF5ZUJeV8AELu4uT8mdec9wI) describing how gating might work.

### Motivation

There is a significant number of slow running tests that have been introduced either unintentionally through bugs in test code or else by being unaware of the ramifications of test delays. An effort is underway to resolve the existing issues, but something should be done to prevent the issues from arising again in the future.

### Impact on Existing Users

Users may have to deal with increased friction when added and updating tests.

### Alternative Implementation

Periodically perform manual audits and attempt to improve tests which perform poorly.

### Additional Context

_No response_

### Notifications

/cc @ampproject/wg-approvers

Start by reading the linked “Keeping AMP Tests Fast” design document and reviewing how AMP tests are currently run and checked. Done means newly created and updated tests are gated against performance and flakiness thresholds, with a documented path for developers to meet those standards.

Written by the indexing model from the issue text.