[BUG] worker test is flaky on firefox

[aslushnikov]

Flaked run: https://github.com/microsoft/playwright/pull/1297/checks?check_run_id=496348690

Test:

playwright/test/workers.spec.js

Lines 30 to 41 in a61d066

	it('Page.workers', async function({page, server}) {
	await Promise.all([
	page.waitForEvent('worker'),
	page.goto(server.PREFIX + '/worker/worker.html')]);
	const worker = page.workers()[0];
	expect(worker.url()).toContain('worker.js');
	expect(await worker.evaluate(() => self['workerFunction']())).toBe('worker function result');
	await page.goto(server.EMPTY_PAGE);
	expect(page.workers().length).toBe(0);
	});

[aslushnikov]

Another test is flaky on Firefox seemengly for the same reason:

https://github.com/microsoft/playwright/pull/1277/checks?check_run_id=496461538