tests: i2c_slave_api: overhaul test implementation

Make the expected data buffers immutable to ensure that the test isn't
modifying them.

Improve clarity by using defines for the two devicetree nodes rather
than repeating the DT_INST() retrieval.

Clean up the naming and diagnostics to more clearly associate the
device label, I2C bus, and I2C address with either EEPROM instance 0
or 1.  Replace the base 1 numbering in some diagnostics.

Document why a device nominally on one bus is being accessed from
another bus.  Return error values from helper functions rather than
invoking ztest failure code so the calling context can provide a
better description of what went wrong.

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