tests: i2c_slave_api: make dual role support optional

Not all I2C controllers that support dual roles allow a controller to
be used in both modes without reconfiguration: for some, registering a
slave device prevents use in master mode.  Refactor so that dual-role
operation is opt-in, and select it for the ST devices currently in the
allow list.

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