tests: drivers: spi: spi_loopback: skip tests if invalid config

The spi_loopback_transceive() helper currently only prints a
message if a configuration is invalid, continuing the test
case as if it succeeded. This results in the test case using the
helper trying to validate the result from a spi transaction that
was skipped.

Fix this by explicitly skipping the test using the ztest
framework's ztest_test_skip() function, which skips the entire
test case.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>