ut logfs: allow flash layer to be destroyed after tests

This was leaking a small amount of memory for every test.

NOTE: This affects the unit test only.
ut logfs: free the fake flash sectors in the mock flash layer

This was causing a sector-sized memory leak for every
simulated sector erase.

NOTE: this affects the unit test only
Merge pull request #229 from solidgoldbomb/fix-ut-memleaks

Fix unit test memory leaks