tests: spi_loopback: Use ZTest properly

Instead of having a bunch of giant if conditions and just running
all the test cases from one actual ZTest case, convert all the test
cases into proper ZTest test cases.

Removing the returns is now required because otherwise there will be
compiler warnings, and they were never doing anything anyways in the
event of zassert fail. ZTest cases are meant to report pass or fail with
ztest paradigm, not with return values and log messages.

Also move the test of the spi lock/release to a separate test suite
since it is not really testing an actual bus transfer, but rather a
that a feature in the SPI API is respected.

Signed-off-by: Declan Snyder <[email protected]>