Add tests for D&O merge strategy

[averevki]

Refined part of #608 with merge strategy tests only

Closes #566

[trepel]

I don't like two additional things:

1. In the bodies of test functions the policy status is not asserted (whether policy is indeed overridden/fully and/or partially enforced).
2. The commit fixture is overloaded so that it only check for policy acceptance. I think (this is philosophical and up to wider discussion maybe) that it is better if commit fixture only deals with policies that are fully enforced (I consider this as initial test setup fixtures should be responsible for because that is what they are designed for) and policies that are overridden/partially enforced (or causes some existing policies to get overridden/partially enforced) should be created in test function bodies (I consider this test scenario rather that initial test setup so it does not belong to fixtures anymore).

[averevki]

In the bodies of test functions the policy status is not asserted (whether policy is indeed overridden/fully and/or partially enforced).

good idea, done

The commit fixture is overloaded so that it only check for policy acceptance. I think (this is philosophical and up to wider discussion maybe) that it is better if commit fixture only deals with policies that are fully enforced (I consider this as initial test setup fixtures should be responsible for because that is what they are designed for) and policies that are overridden/partially enforced (or causes some existing policies to get overridden/partially enforced) should be created in test function bodies (I consider this test scenario rather that initial test setup so it does not belong to fixtures anymore).

agreed, there are a lot of other tests doing that inside the testsuite though. We can do another single PR with all these changes in the future

[trepel]

LGTM now. Thanks!