Improve signmessages functional test
Summary:
This patch improves branch coverage of the test, making sure a message can not be verified with the wrong address or signature.
Backport of core PR11241
https://github.com/bitcoin/bitcoin/pull/11241/files
Test Plan:
./test/functional/test_runner.py rpc_signmessage
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3410