[tests] [qt] Introduce qt/test/util with a generalized ConfirmMessage
ConfirmMessage is reused in future tests apart from its single usage here.
This is a partial backport of Core PR12830 : https://github.com/bitcoin/bitcoin/pull/12830/commits/9c01be1b85f8ad11a8a0826a4a2bcdc2668a5c1f
Because ConfirmMessage was deleted - as we don't need it, it has to be recreated rather than moved. However, it is later used in PR12830 so we need it back.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3666