test: add page.bringToFront test

[kblok]

I think we can test page.bringToFront in headful.

[JoelEinbinder]

This is cool, and looks like it works. I am concerned that it is more testing for the presence of a tab strip than for bring-to-front to be actually working properly. Browsers don't need to have a tab strip that behaves this way, with only one web contents visible at a time, nor do they need to make the page created by browser.newPage() visible. But maybe that's fine, and we can cross that bridge when we get to it. @aslushnikov wdyt?

[aslushnikov]

Browsers don't need to have a tab strip that behaves this way

Yeah, but I'd speculate that browsers are unlikely to change here in the foreseeable future.

nor do they need to make the page created by browser.newPage() visible

I don't really think we've relied on this anywhere so far. Let's not use this assumption in the test:

const page1 = await browser.newPage();
const page2 = await browser.newPage();

await page1.bringToFront();
expect(await page1.evaluate(() => document.visibilityState)).toBe('visible');
expect(await page2.evaluate(() => document.visibilityState)).toBe('hidden');
await page2.bringToFront();
expect(await page1.evaluate(() => document.visibilityState)).toBe('hidden');
expect(await page2.evaluate(() => document.visibilityState)).toBe('visible');

But maybe that's fine, and we can cross that bridge when we get to it

Yeah I like it! Let's land this!

[kblok]

Done!