tests: i2c_slave_api: update to current standard terminology

Zephyr does not currently allow deviation from standard terminology
for a technology even if it is non-inclusive, until the corresponding
standards body has confirmed intent to change that terminology.  The
terms used in a previous attempt to be inclusive do not match the
expected forthcoming standard terms.

Revert to standard terms until the new ones have been announced and
the switch made throughout Zephyr.

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