[Feature Request] Forbid the use of Thread.sleep in tests

Is your feature request related to a problem? Please describe

Using a fixed sleep in tests is a great way to introduce a flaky test. It's almost always better to poll for the condition you're waiting for rather than waiting a fixed amount of time. Here are a handful of flaky tests from the past that were caused by fixed sleeps:

• #19422
• #14288
• #18820

This list is certainly not exhaustive.

Describe the solution you'd like

Use forbidden APIs to forbid the usage of Thread#sleep in test code. There are certainly cases where sleeping will still be needed so this can be overridden with the @SuppressForbidden annotation, but making the default behavior to fail the build will make it clear to contributors and reviewers that any usage of sleep needs scrutiny.

Additional context

Because usage of Thread.sleep is currently quite pervasive, my rollout plan for this is to chip away one at a time by making it an optional property that can be turned on one test suite at a time. For example, the following can be added to server/build.gradle to enable this for internal cluster tests:

tasks.named('forbiddenApisInternalClusterTest').configure { forbidSleep() }

This will avoid a huge PR to annotate or remove hundreds of usages across all the projects. Once we reach a critical mass we can switch the default in this repo.

Start with server/build.gradle and the forbiddenApisInternalClusterTest configuration, then trace how forbidSleep() is enabled for a test suite. Done means enabled suites reject Thread.sleep usage by default, while cases marked with @SuppressForbidden remain possible and rollout can proceed one suite at a time.