osd: Add unittests for PeeringState

osd: Add unittests for PeeringState

Add test harness for testing PeeringState. Add mock classes
for PeeringListener, PGBackendListener and PGBackend to allow
instances of PeeringState to be created. Test harness to pass
peering messages between OSD instances, configure the OSDMap
with a pool and one PG. Some basic initialization unittests
plus tests that run the whole peering process to active+cleam
including testing rollback of incomplete writes in an EC pool,
recovery, async recovery and backfill.

Includes tests for issues 71493, 73891 and 74218

AI: AI assisted in writing the mock classes providing stubs
for all the virtual functions and writing the simpler unittests.
Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>