tests: kernel/events/event_api: memory coherence fixes

When CONFIG_KERNEL_COHERENCE=y, the k_event struct objects
cannot be declared on stacks since they are incoherent among
CPUs in the system.

So mark them as static to place them in global data section
and thus are coherence between CPUs.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>