os/bluestore/bluefs: Add test that detects bluefs inconsistency

os/bluestore/bluefs: Add test that detects bluefs inconsistency

Add test that detects possible scenario that will cause BlueFS to have file
that contains data that has never been written. This is done by tricking
replay log to already accept file metadata (size, allocations), but actual data
stored in these allocations is not yet synced to disk.

Scenario:
1) write to file h1 on SLOW device
2) flush h1 (and trigger h1 mark to be added to bluefs replay log)
3) write to file h2
4) fsync h2 (forces replay log to be written)

The result is:
- bluefs log now has stable state of h1
- SLOW device is not yet flushed (no fdatasync())

Test detects this condition and fails.

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>