Allow known flaky tests to be automatically repeated

Problem/Motivation

#3037436: [random test failure] Make QuickEditIntegrationTest more robust and fail proof → and 🐛 [random test failure] Random failures building media library form after uploading image (WidgetUploadTest) Closed: cannot reproduce amongst others are wasting contributor time as they knock RTBC patches back to Needs Work, when in fact it was just a random test failure. These failures are false positives and always go away on a retest.

Steps to reproduce

Mark a patch RTBC and wait a few days/weeks for a random test failure.

Proposed resolution

Provide a way of marking a test as "flaky". If a flaky test fails, automatically repeat it in the test run and only actually fail if it fails the second time round.

Remaining tasks

Decide if this is a good idea.
Figure out how to implement it.

User interface changes

API changes

Data model changes

Release notes snippet