tstest: coalesce dropped Ticker ticks when advancing the clock far ahead

Clock.Advance previously fired each Ticker once per period between the
current time and the advance target, even though all but the first few
sends were dropped on the Ticker's full channel. A test that advances
the clock 180 days with a 5 second ticker registered did 3.1 million
fires, each taking a mutex, attempting a channel send, and fixing the
event heap. Under the race detector, where synchronization operations
are 20-30x more expensive, this dominated test runtime.

Now, when a tick is dropped because the channel is full, skip the
remaining missed ticks in one step, staying aligned to the original
schedule and still firing the final tick at or before the advance
target. This matches time.Ticker, which drops ticks it cannot deliver,
and is observably identical behavior. Ticks that fit in the channel are
still all delivered.

In the internal corp repo, this takes controlclient.TestExpiry under
race from 6.0s to 2.2s (its no-race time is 0.5s), and the new
benchmark improves from 449µs to 90ns per 24h advance:

  BenchmarkTickerLargeAdvance-16  100       449313 ns/op    (before)
  BenchmarkTickerLargeAdvance-16  13311910      90.22 ns/op (after)

Updates tailscale/corp#47035

Change-Id: I4826df212415eb2ea7ceb5bc46848e816f8ccde1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>