test: fix timeout code for invoking compiler

When running go tool compile,
go tool is running compile as a subprocess.
Killing go tool with Process.Kill leaves the subprocess behind.
Send an interrupt signal first, which it can forward on
to the compile subprocess.

Also report the timeout in errorcheck -t.

Change-Id: I7ae0029bbe543ed7e60e0fea790dd0739d10bcaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/282313


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>