Addresses #12796.
When a user attempts to add to the address book a sending address which is already present as a receiving address, they’re presented with a confusing error indicating the address is already present in the book, despite the fact that this row is currently invisible.
This change adds a more specific error message indicating its existence as a receiving address (as discussed in the linked issue).
This change also adds some tests exercising use of the address book via QT. Adding so much test code for such a trivial change may seem weird, but it’s my hope that this will make further test-writing for address book usage (and other QT features) more approachable.