Bug 1733934 [wpt PR 31094] - taskcluster: Raise the max time per chunk for testharnes...

Bug 1733934 [wpt PR 31094] - taskcluster: Raise the max time per chunk for testharness to 3h., a=testonly

Automatic update from web-platform-tests
taskcluster: Raise the max time per chunk for testharness to 3h.

Commit 6f7c2fd96f raised the number of chunks from 16 to 17 in an
attempt to avoid the timeouts in the WebKitGTK runs (see #30834),
but this didn't worked.

The issue seems to be caused because several tests that get scheduled
in the same chunk (because of living in the same directory).
This tests (around 50) timeout, taking each one 1 minute to timeout,
so they consume almost 1hour from the chunk doing nothing.

So let's try to raise the timeout to 3h to avoid hitting the 2h time
limit. I tested locally that 3h should be enough for this to pass.

Hopefully we can fix this in a better way soon, but in the meantime
let's apply this measure to get the runs back working ASAP.

This patch also lowers back the number of chunks to 16, as raising
them doesn't seem to be helpful for fixing the issue.

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wpt-commits: a218d10eb96cf7e29751f39593f06c3ea359cc21
wpt-pr: 31094