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
  becomes 0.
- move random R/W and sequential update into common code to share

Change-Id: I31f37b40322c2bca94942c2cb7500799b19bed0d