tests: spi: spi_loopback: rename SPI_SLAVE to SPI_SLAVE_NUMBER

As the SPI_SLAVE define is used by some HAL (e.g. the gigadevice HAL),
SPI_SLAVE has been renamed to SPI_SLAVE_NUMBER.

Signed-off-by: Sebastien Bourdelin <sebastien.bourdelin@rtone.fr>