tests: lib: spsc_pbuf: Do not run test with cache

Test was using a configuration which enforces cache management in the
packet buffer. However it shall not be used if producer and consumer
is the same core. Testing this configuration on a single core does
not make sense as it actually fails on cores with data cache.

Making this configuration build_only so it is checked against
compilation errors.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>