prober: deflake TestProberConcurrency

The fake ticker has a one-element channel buffer and drops ticks when
the probe loop goroutine isn't already blocked on the channel, so
advancing the fake clock 50 times in a tight loop didn't guarantee
that the loop observed enough ticks to start three concurrent probe
runs. Under CI load, only two of the three run goroutines could be
spawned before the convergence timeout expired.

Advance the clock inside the polling loop instead, so ticks keep
firing until all three probe goroutines have started. Verified with
flakestress: the old test failed within ~41k runs, while the fixed
test passed 175,214 runs with no failures.

See http://flakes/analyze-test?name=tailscale.com%2Fprober.TestProberConcurrency

Updates #deflake

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I673a4918bbb5fea6b650e0dc1bc491c4af922b19