tools/lib/testutil: increase race test timeout to 15mins
tools/lib/testutil: increase race test timeout to 15mins

device impl test times out occasionally at the 10m mark on Jenkins. Increasing
the timeout will allow us to figure out if it's a slow test issue, or if there's
actually something hanging.

Change-Id: Id1a92a58ce7aea62dd3e26e773066993f10eb24f

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools