Bluetooth: Mesh: Increase BSIM test timeout to 5 minutes

The wallclock time timeout of the test execution is limited to let CI
abandon hanging test cases. Increase this timeout to account for
additional tests running in parallel in CI, which has caused enough
resource contention to make CI break the time limit.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>