AllocationCandidatesTestCase includes tests using get_by_requests().
However, it also includes two tests for specific functions,
test_get_trees_matching_all() and test_get_trees_with_traits().
This patch separates the two tests creating a new test class,
ProviderTreeDBHelperTestCase.
Change-Id: I83fc00a96353b985aa2aa60fb146329dd2f4e767