tests: spi_loopback: Fix same_buf_cmd test

The tests was written wrong. It was meant to test using same spi bufs
for both rx and tx, as in tree many sensor and other spi device drivers
use this paradigm. But the 2nd buf setup call was overwriting the first.
Fix by not using the helper function for this case.

And for the write back test, test using same spi_buf_set.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>