These changes make it so that a set of common test cases can be used for
all merge strategies, with specific test cases being able to be
specified on a policy-by-policy basis.
This abstraction moves the responsibility of merging topology hints to
the individual policies themselves. As part of this, it removes the
CanAdmitPodResult() API from the policy abstraction, and rolls it into a
second return value from Merge()
Previously it only took a bool, which limited the logic it could perform
to determine if a pod should be admitted or not based on the merged hint
from the policy.
