tests: drivers: spi: loopback: Add test for pico-spi-pio driver

Plus refactored the structure of the overlay files.
The wiring of the new test is the same as the existing spi tests
for rpi-pico.

The speed of the fast test has to be reduced to ensure signal integrity.

Setting CONFIG_SPI_ASYNC will trigger a hard fault. Seems to be a bug in
the driver.

Signed-off-by: Terry Geng <terry@terriex.com>