tests: drivers: spi: Add test for SPI master and slave

Add test which is using one SPI master and one SPI slave instance.
There is already a loopback test for SPI master so this test is
focusing on SPI slave API. Test requires 4 pairs of GPIO pins
shortened together. One set of pins is used by SPI master and
one for SPI slave.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>