test/osd: Add scrub to osd test fixtures

test/osd: Add scrub to osd test fixtures

Add scrubbing functionality to the OSD test fixtures to enable testing
of scrub behavior on erasure-coded pools. This implementation uses as
much of the scrub backend as possible, but avoids using pg_scrubber or
the interaction with peering due to the complexity of the dependency on
PG.

Changes:
- Add scrub_object() method to PGBackendTestFixture for testing scrub
  functionality on EC pools
- Add create_random_buffer() utility for generating random test data
- Add corrupt_shard_data() method to intentionally corrupt shard data
  for testing scrub detection
- Add MockScrubBeListener, MockPgScrubBeListener, MockSnapMapReader, and
  MockLoggerSinkSet mock classes to support scrub testing
- Add three new test cases: ScrubClean, ScrubDetectsCorruption, and
  ScrubPartialWrite to verify scrub behavior with clean data, corrupted
  data, and partial writes

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
# Conflicts:
#	src/test/osd/TestECFailoverWithPeering.cc