test: Relocate posix mqueue receive buffer

For platforms that select Kconfig option CONFIG_KERNEL_COHERENCE,
the receive buffer used in mq_timedreceive() must not be on the
stack. This is because the k_msgq that underlies the POSIX mqueue
will attempt to perform a direct to buffer copy from the sender
when there is a waiting reader.

Fixes #60796

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>