tests/functional/test_migration: Use "ncat" instead of "nc" in the exec test

tests/functional/test_migration: Use "ncat" instead of "nc" in the exec test

"nc" can either be GNU netcat, OpenBSD netcat or NMap ncat. At
least GNU netcat currently does not work with this test anymore,
though the comment in the test says otherwise. GNU netcat seems
to be quite unmaintained nowadays, according to its website
(https://netcat.sourceforge.net/), the last public release is from
2004, so we should rather avoid that binary.
In our CI, we are only using "ncat" in the containers (it's the only
flavor that lcitool supports), thus to avoid silent regressions with
the other netcats, let's limit this test to "ncat" only now.

Reported-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Message-ID: <20250319105617.133191-1-thuth@redhat.com>
Reviewed-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Thomas Huth <thuth@redhat.com>