os/exec_test: fix test on Plan 9
Error strings vary across OSes when trying to execute a file
that does not exist. Since matching them is not the point of
the test, ignore them.
Fixes #76965
Change-Id: I6d220bc2d0289070f3441adb48983c13b2a3e597
Reviewed-on: https://go-review.googlesource.com/c/go/+/732940
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>