Test UI: Ability to `preserveFocus` when running tests

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> In our use of the Testing API, the `testing.followRunningTest` setting (default is `true`) needs to be set to `false` or else the Testing UI flickers, scroll position changes, and focus changes lead to a confusing user experience. I understand the need for this feature but believe that extension writers should have the ability to opt-out of the behavior for their context with a `preserveFocus` setting, similar to other places: * `window.createWebviewPanel` * `window.showTextDocument` * `OutputChannel.show` * `Terminal.show` Maybe this could be exposed via an extra property of a `TestRunRequest`?