Don't poll if the selected test is still selected. Wait for a response from the websocket instead that the test has started (that updates item prop). This polling fetch would fail (sw errors), causing the model to be set to a new TraceModel with an empty traceURI, which in case would make the source call fail (the original issue).

Added assertions to existing test.

Tried instead adding following assertions in runUITest fixture:

expect.soft((page && !page.isClosed()) ? await page.consoleMessages() : []).toHaveLength(0);
expect.soft((page && !page.isClosed()) ? await page.pageErrors() : []).toHaveLength(0);

However many would then fail:
snapshot action must have a pageId (10+ ui-mode tests; e.g. should not duplicate network entries from beforeAll)
Failed to load resource: the server responded with a status of 404 (Not Found) (should run folder / should merge trace events / should merge web assertion events / should show request source context id)

Closes: #38359