[441172] Fix merge order of equivalent diffs for one-to-one opposites

When determining which one of the equivalent differences of one-to-one
eOpposites will be merged above the others, differences which result in
setting a feature value instead of unsetting a feature will be
preferred.

Includes testcases for both merge directions.

Bug: 441172
Signed-off-by: Stefan Dirix <st.dirix@gmail.com>
Change-Id: I2e98f44e5e440c82ec95135db622225c65dc07b3