Fix AugmentationIdentifier.compareTo()

Fix AugmentationIdentifier.compareTo()

The implementation assumed stable Set iteration order, which was
not internally guaranteed, hence two AugmentationIdentifiers could
report as being equal via equals(), but still compare as having
different total ordering -- which could lead to strange results.

Fix this by doing some more work and compare them based on QName's
total ordering. The test suite is fixed up to use explicit iteration
order retained by ImmutableSet rather than relying on HashSet's
hashCode()-based iteration order non-determinism.

Also remove implicit boolean boxing by using assertTrue/assertFalse
in tests.

Change-Id: Iafd4ac8d7d54f0918b0579c6e6f8e76b35b56dd8
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>