Add bitcoin-qt testing explicitly to the releease process

Summary:
Multiple times in the past, bitcoin-qt has either been broken in a release or had very close calls.
Calling this manual testing step out in the release process should reduce our failure rate until we're able to
automate this process better.

Test Plan: Read it, try it.

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Fabien, Mengerian, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2581