tests: i2c_slave_api: add node labels to devicetree

The test currently identifies devices by instance number, which is not
guaranteed to be constant across builds.  Add node labels so the test
can be updated to use a more permanent node identifier.

Also remove insensitive (and misleading) terminology in the node
labels.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>