[snipmatch] Bug 451357: Extract and test ProposalSorter

Change-Id: I694c450bb8b8f82f3df43eae64c4dd2fe1806b4f
Signed-off-by: Johannes Dorn <johannes.dorn@codetrails.com>
Signed-off-by: Andreas Sewe <andreas.sewe@codetrails.com>