tests/unit/test-iov: Fix timeout problem on NetBSD and OpenBSD

tests/unit/test-iov: Fix timeout problem on NetBSD and OpenBSD

The test-iov code uses usleep() with small values (<= 30) in some
nested loops with many iterations. This causes a small delay on OSes
like Linux that have a precise sleeping mechanism, but on systems
like NetBSD and OpenBSD, each usleep() call takes multiple microseconds,
which then sum up in a total test time of multiple minutes!

Looking at the code, the usleep() does not really seem to be necessary
here - if not enough data could be send, we should simply always use
select() to wait 'til we can send more. Thus remove the usleep() and
re-arrange the code a little bit to make it more clear what is going
on here.

Suggested-by: "Daniel P. Berrangé" <berrange@redhat.com>
Message-ID: <20240122153347.71654-1-thuth@redhat.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>