[Feature] Add ability to combine parallel and serial tests

[OSA413]

I'd like to see an option to combine serial tests and parallel tests. That would allow following scenario:

1. Do one test (that in sequence)
2. Do a bunch of parallel tests after the first test
3. Do another (single) test in the sequence
4. Finish with another bunch of parallel tests that run after all previous tests

Here's a draft of what I'd like to accomplish, to have parallel tests after some required test:

But the describe block marked with parallel doesn't show up in the UI mode (and won't run)

And if I remove the parallel in the second describe block tests appear, but they run in sequence

[OSA413]

Managed to do first two steps with replacing the very first test with

test.beforeAll("setup test", async ({ browser }) => {
        const page = await browser.newPage();
        ...
}

and removing serial in the test description block.

I'd prefer a less hacky way of doing it

[dgozman]

Folding into #16467.