package/libiscsi: add optional dependency to cunit

package/libiscsi: add optional dependency to cunit

libiscsi uses cunit for its test suite, and autodetects its presence.

Usually, we just try to disable tests altogether, but there is no way to
do so. So, ensure proper ordering.

Note: there is an ac_cv_have_cunit variable, but it is not a true
ac_cv* cache variable, and the value provided on the command line or
environment is properly ignored.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>