[piexwasm] Make runTest return its Promise

Return a Promise from runTest so test.js will actually await it. Note:
helps to reduce jitter in our timing measurements, since tests.js will
no longer be banging on the renderer for document.title == 'DONE'.

Bug: 935285
Change-Id: Ibfae52c755587104f447e936c1864a995cdf71e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1568366
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651133}
2 files changed