tests: No thread-shared/synchronized data on stack (coherence)

CONFIG_KERNEL_COHERENCE forbids synchronized data on the stack: no
spinlocks, IPC primitives, or things that contain them.  Application
code obviously doesn't have to follow these inconvenient rules, but
our test code needs to run on platforms with incoherent stack memory.

Make these things static.

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