tests: drivers: spi: spi_loopback: add fixture spi_loopback
tests: drivers: spi: spi_loopback: add fixture spi_loopback

Add fixture spi_loopback.

Signed-off-by: Alexandre Bourdiol <c23cc711c078d3e6bb50b3dc45a1b8842691e59a@st.com>

zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr