x11-toolkits/qt5-declarative: split out 'test' parts into new port

kde@ has been focused recently on reducing the load of "unnecessary"
runtime dependencies, to allow third party distributions to have a
leaner installation.
One part of this effort was to make qt5-testlib a "build only"
dependency.
Unfortunately, parts of qt5-declarative link against testlib (to provide
test functionality for declarative parts).

This lead to an unfortunate loop of pkg-autoremove removing testlib, and
then pkg-update adding it again (as it was an improperly registered
dependency).

This change splits out the "test"-parts into its own port. Thereby
making testlib not even a required build-time dependency for
x11-toolkits/qt5-delcarative.

Reported by: jrm
Differential Revision: https://reviews.freebsd.org/D32097