tests: drivers: spi_loopback on stm32f3_disco

Configure through an overlay file, the stm32f3_disco board
for running the tests/drivers/spi/spi_loopback.
The SPI1 is selected: MOSI/MISO output pins are available
on the board.
Connect pin PA6 & PA7 of connector P1 to PASS the test.
Add the spi_loopback fixture.

Signed-off-by: Francois Ramu <francois.ramu@st.com>