Add timeout to Heap snapshot tests
Add timeout to Heap snapshot tests

The heap snapshot UI is large and renders a lot of data. When running
the tests locally with `STRESS=1` (to emulate slower bots vs my MacBook)
they routinely fail. Upping the timeout to 15 seconds eliminates the
failure (tested via multiple `it.repeat` runs). I also tested a 10
second timeout, but that wasn't long enough.

Bug: 1110817
Change-Id: I88d44e5e7a3c6119cdf9b28b1774ebc102439ea4
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2410220
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Reviewed-by: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>

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