Bug 8300 - Add Test::WWW::Mechanize as an optional dependency

Bug 8300 - Add Test::WWW::Mechanize as an optional dependency

This patch replaces an earlier patch by Marcel de Rooy, which
had become outdated because lots of new dependencies were
added since the patch was made.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
(cherry picked from commit 508009c7a836375aaf2a5e18db10706ec73cff25)

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>