Fix immutable NormalizedNode equality

Fix immutable NormalizedNode equality

Equality has always been iffy in our immutable implementations, but we
are now making a decent attempt to implement it. There are three gaps in
that effort, where we only consider known subclasses for comparison of
value part of NormalizedNode.

Address this by providing fallback child comparison and add an explicit
test suite.

JIRA: YANGTOOLS-1417
Change-Id: I36e5bcfc6da06c789595b00b7ad86d973cb34703
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit d7afd8f9c3dd23433656663dbcf536b8a898f5d1)