iotests: fix copy-before-write for macOS and FreeBSD

iotests: fix copy-before-write for macOS and FreeBSD

strerror() represents ETIMEDOUT a bit different in Linux and macOS /
FreeBSD. Let's support the latter too.

Fixes: 9d05a87b77 ("iotests: copy-before-write: add cases for cbw-timeout option")
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220705153708.186418-1-vsementsov@yandex-team.ru>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>