Use H2 in-memory database in SQLKeyValueStoreTest.

Change-Id: Ib6d33690a71c7ebb8051945c85b4b3e4aef52368
Reviewed-on: http://naan.mimuw.edu.pl:8001/107
Tested-by: Nebulostore Nebulostore <nebulostore@gmail.com>
Reviewed-by: Lukasz Siczek <siczek.lukasz@gmail.com>