[BUG] http credentials test is flaky on firefox linux

[aslushnikov]

Flaky run:
https://github.com/microsoft/playwright/pull/1301/checks?check_run_id=496478707

Test:

playwright/test/browsercontext.spec.js

Lines 385 to 400 in cf46f1b

	it('should fail if wrong credentials', async({browser, server}) => {
	server.setAuth('/empty.html', 'user', 'pass');
	const context = await browser.newContext({
	httpCredentials: { username: 'foo', password: 'bar' }
	});
	const page = await context.newPage();
	let response = await page.goto(server.EMPTY_PAGE);
	expect(response.status()).toBe(401);
	await context.setHTTPCredentials({
	username: 'user',
	password: 'pass'
	});
	response = await page.goto(server.EMPTY_PAGE);
	expect(response.status()).toBe(200);
	await context.close();
	});