test-suite: Separate generators and tests

Declutter Properties.hs by moving generators into Authochan.Tests.Gen
and message verification tests into Authochan.Tests.Message. This would
have to be done sooner or later as new features and tests are added to
authochan.

Signed-off-by: Stian Ellingsen    <stian@plaimi.net>
Reviewed-by:   Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Add sample messages and signatures to messageTests

The sample messages are checked against precalculated signatures,
guarding against erroneous changes in message serialisation or hashing,
and will help client developers make sure that they generate correct
signatures.

Signed-off-by: Stian Ellingsen    <stian@plaimi.net>
Reviewed-by:   Alexander Berntsen <alexander@plaimi.net.
Signed-off-by: Alexander Berntsen <alexander@plaimi.net.