os/exec: add temporary debugging code for #25628

os/exec: add temporary debugging code for #25628

On linux-386 builders run the TestExtraFiles subprocess under strace,
in hopes of finding out where the unexpected descriptor is coming from.

For #25628

Change-Id: I9a62d6a5192a076525a616ccc71de74bbe7ebd58
Reviewed-on: https://go-review.googlesource.com/c/go/+/225799
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>