tests/kernel: Coherence: no shared data on stacks

A fairly common idiom in our test code is to put test-local data
structures onto the stack, even when they are to be used from another
thread.  But stacks are incoherent memory on some platforms, which
means that such things may not get a consistent view of memory between
threads.

Just make these things static.  A few of these spots were causing test
failures on intel_adsp_cavs15.  More were found by inspection while
hunting for mistakes.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>