support/script/scancpan: add -test option

Perl modules can have three different types of dependencies:

 - configure/build time dependency which becomes host dependency
 - runtime dependency which becomes target dependency
 - test time dependency which is useless in a cross-compiling context like BR

Before this patch, test time dependencies are handled like runtime
dependencies.

After this patch, test time dependencies are ignored by default. The
newly added -test option allows to add them anyway if needed.

[Thomas: reword commit log using Francois proposal.]

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>