tests: spi_loopback: Fix high RAM usage

Display diff of contents for large buffers is not quite helpful and
takes up huge RAM, and if a board has less RAM then this causes the test
module build failures.

So, disable display of diff and just log a failure, small buffer tests
can be used to debug such basic issues and large buffer tests can act as
a smoke test for debugging other issues. This saves about 80K of RAM.

Fixes #72792.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>