Use in-memory sqlite DB for unit tests

Currently unit tests use temporary file for DB. This causes
several problems:
1. temp files need to be cleared even after crash
2. temp folder need to be properly configures in OS
3. temp folder could be modified in code (e.g.
   oslotest.base.BaseTestCase)

Using in-memory DB fixes all these problems.

Partly implements: blueprint oslotest

Change-Id: I8be76efdb2fa56f5e8865dcafbc3c240a6be0f64
Merge "Use in-memory sqlite DB for unit tests"