testing: print late arriving log line in panic

When you log after a test has completed,
the testing package panics.

Print the logged line as part of that panic,
to aid in debugging.

Change-Id: I3d6689d1eed57c03e300afe37db0c15b2f4acda4
Reviewed-on: https://go-review.googlesource.com/c/go/+/283972


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>