tests: Ship a test URI scheme handler

Include a stub URI scheme handler for the `didcot-test:` scheme in the
didcot-tests package, to avoid depending on a web browser being
installed.

Apertis: https://phabricator.apertis.org/T2702



Signed-off-by: Rohan Madgula <Rohan.Madgula@in.bosch.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D6114