If the test has already completed when a go routine with a panic
handler reports an error the location of the error call is lost.

Added logDepth to be able to log location of failure at different
depths down the stack.

Fixes #26720

Change-Id: I8b7789ddae757ef6f4bd315cb20356709f4fadec
Reviewed-on: https://go-review.googlesource.com/c/127596
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>