Bump CI timeout up to 300s
Bump CI timeout up to 300s

Tests are slow when run with -race on PotatoCPU(tm), such as the one that travis
uses :).

R=dnj@chromium.org, estaab@chromium.org, maruel@chromium.org, vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1440823002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go