log: avoid leaking goroutines in TestOutputRace

log: avoid leaking goroutines in TestOutputRace

Leaked goroutines are the only explanation I can think of for excess
allocs in TestDiscard, and TestOutputRace is the only place I can see
where the log package leaks goroutines. Let's fix that leak and see if
it eliminates the TestDiscard flakes.

Fixes #58797 (maybe).

Change-Id: I2d54dcba3eb52bd10a62cd1c380131add6a2f651
Reviewed-on: https://go-review.googlesource.com/c/go/+/487356
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>