pluginloadtest and sendfiletest are crashing. This patch fixes that by allowing Daemon::init() to be called from TestDaemon
## Test Plan
### Before:
Both tests are crashing because it is not able to find any devices. It is not able to find any devices because the LanLinkProvider is not being added because Daemon::init() is not being called.
### After:
pluginloadtest and sendfiletest pass
Summary:
Add a helper function to the daemon interface, which must be subclassed
by the implementations, which creates a simple notification.
For more complex needs, involving interacting with the notification, it
will still be nescessary to use KNotification directly, but this allows
for other future implementations to create a simple notification without
ifdef'ing the code.
Reviewers: kdeconnect, apol
Reviewed By: apol
Subscribers: kdeconnect
Differential Revision: https://phabricator.kde.org/D11104
