add more cases to AlmostFull case
- move disk fill into setUp stage and adjust free space to 1GB
: < 500MB kicks in cache erase multiple times, which can affect the reult.
- share disk filling across all tests: use the property that TestCase instances
are created as a batch at the beginning -> add reference counting to prevent
filling again and again. The filled disk is cleared when reference count
- move random R/W and sequential update into common code to share
5 files changed