test_channel: keep time constant when running channel/outbound_cell

With fairly low probability, the outbound_cell test could get run with cached_gettimeofday on a different 10-second boundary than the current value of gettimeofday, resulting in a call to scale_active_circuits().

Now that we no longer use cached_gettimeofday, the probability of calling scale_active_circuits is even lower here. If the test takes about 200 microseconds (as it does on my desktop), we should only expect it to straddle the 10-second boundary with P < 200 usec / 10s == 1 / 50000.

Still, it's a good idea to fix this kind of thing.

This change will lower coverage from the maximum by making it so this test does not call scale_active_circuits. However, this test does not actually validate any of the behavior of scale_active_circuits, so this change is appropriate.