os/exec: ignore context.Canceled errors in TestConcurrentExec

os/exec: ignore context.Canceled errors in TestConcurrentExec

We cancel the Context to unblock the test as soon as all of the "exit"
processes have completed. If that happens to occur before all of the
"hang" processes have started, the Start calls may fail with
context.Canceled.

Since those errors are possible in normal operation of the test,
ignore them.

Fixes #61277.
Updates #61080.

Change-Id: I20db083ec89ca88eb085ceb2892b9f87f83705ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/508755
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>