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>