test_metadata_agent: don't check implementation details

test_metadata_agent: don't check implementation details

TestUnixDomainMetadataProxy tests too many implementation details when
checking cases of existing and non-existing socket file. Reduce mock
checks to the minimum needed to test for expected behaviour.

Otherwise test may catch wrong isdir() calls (f.e. when policies are
initialized) and fail (I experienced it in local checkout).

Change-Id: I0f85730e5611a71290542f23f8bdcaaaadbef42c