Add unit-test assert to check dict is superset of dict

Add unit-test assert to check dict is superset of dict

Several VPN unit tests need the ability to check that an expected
dict is found in the actual dict output (and that other elements are
ignored - like auth tokens).

Since this will be of use to other components, adding this to the
base test case.

Note: There is a similar assertDictContainsSubset(), however, it is
not supported in 2.6 and has been deprecated in 3.2. A different, and
hopefully clearer, name is used for this method.

Change-Id: I8becf9cfad966874f8463a9c835331f2f17d2ead
Closes-Bug: 1366797