It's not possible to calculate the total test duration of the tests in
the UI, since our `msToString` function is lossy. This patch unflakes
the
[test](https://github.com/microsoft/playwright/actions/runs/6183832112/job/16787806162?pr=27074#step:7:254)
when it took longer than a second to start, by writing the test-ids to
the DOM, getting them into the Node.js process, and calculating their
test duration sum based on the JSON report.
Drive-by: I found a bug that we used the JSON report from the main
test-runner process rather than from the merge process, so the test IDs
were still old. (Without that change the `useIntermediateMergeReport`
tests were not passing.
aa929bff3b