tests: ipc: pbuf: Disable data cache management

Packet buffer is designed to run writer and reader code on two CPUs.
The write function cannot be preempted by the read function in a CPU,
or the cache management could result in data corruption. In the tests,
the reader and the writer are executed on a single CPU. Disable data
cache management to prevent potential data corruption.

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