Hyper-V: Adds Hyper-V SnapshotOps tests

The Hyper-V driver oldest tests (test_hypervapi.py) are proving hard to
maintain.

The tests in test_hypervapi.py in particular can also be refactored and
split in separate TestCases, one of each *ops module.

This commit refactors the SnapshotOps test cases.

Co-Authored-By: Robert Tingirica <rtingirica@cloudbasesolutions.com>

Partially implements: blueprint hyper-v-test-refactoring

Change-Id: I5e6cf64d47959cf348b4cacfa931da8630a995ee
Merge "Hyper-V: Adds Hyper-V SnapshotOps tests"