Fix ChoiceNodeContextNode's byQName indexing

Fix ChoiceNodeContextNode's byQName indexing

When we are traversing by provided QName, we are meant to encounter
each ContextNode significant to PathArgument construction. This works
for all cases except for 'choice->case->choice', where
ChoiceNodeContextNode's internal indexing does not account for itself:
it ends up storing the second ChoiceNodeContextNode under the
ChoiceSchemaNode's QName -- and therefore it fails to find it by its
consituents' QNames.

This has impact primarily on instance-identifier parsers, as evidenced
by the corresponding JSON-based unit test.

Fix this by properly overriding getQNameIdentifiers() to provide the
internal table's keySet().

JIRA: YANGTOOLS-1411
Change-Id: I11df2d4072157e06342c2899dbcf02f74eef8090
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>