[Why]
Once `khepr_db` is enabled by default, we need another way to disable it
to select Mnesia instead.
[How]
We use the new relative forced feature flags mechanism to indicate if we
want to explicitly enable or disable `khepri_db`. This way, we don't
touch other stable feature flags and only mess with Khepri.
However, this mechanism is not supported by RabbitMQ 4.0.x and older.
They will ignore the setting. Therefore, to make this work in
mixed-version testing, we set the `$RABBITMQ_FEATURE_FLAGS` variable for
the secondary umbrella. This part will go away once we test against
RabbitMQ 4.1.x as the secondary umbrella in the future.
At the end, we compare the effective metadata store to the expected one.
If they don't match, we skip the test.
While here, change `rjms_topic_selector_SUITE` to only choose Khepri
without specifying any feature flags.
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