Fix stack mismatch in data change event streams

Fix stack mismatch in data change event streams

Data change event stream targeting a list instance requires a special
treatment to avoid mismatch between the writer stack and data.
Wrap the map entry in a map node to avoid the stack mismatch and
serialization error.

Also, add test cases to cover the use case to ListenerAdapterTest and
refactor/clean up the test class to reduce duplicate code and make file
& variable names more consistent.

JIRA: NETCONF-1051
Change-Id: I0e266f04fc77db6add4f1c44e6fd47f208c2e2f9
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>