os/exec: use subprocess deadline in TestExtraFiles
os/exec: use subprocess deadline in TestExtraFiles

Try to get some output even if the subprocess hangs.

For #25628

Change-Id: I4cc0a8f2c52b03a322b8fd0a620cba37b06ff10a
Reviewed-on: https://go-review.googlesource.com/c/go/+/227517
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

nawawi/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,nawawi/go,golang/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go