Bug 18243: 16034 follow-up: Conditionally test Koha::ExternalContent::* modules loading

Bug 18243: 16034 follow-up: Conditionally test Koha::ExternalContent::* modules loading

To test:
1 - Make sure you don't have WebService::ILS installed
2 - prove t/00-load.t
3 - You should get a failure to load module and tests are skipped
4 - Apply patch
5 - prove t/00-load.t
6 - Module is not loaded, test are skipped
Optional:
7 - Install WebService::ILS
5 - prove t/00-load.t
6 - Module is loaded, test passes

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>