tests/kernel: Add test for CONFIG_SMP_BOOT_DELAY

This feature is in tree and used by the SOF app, but we don't have a
local test for it.  Add one, including a case to track regressions in
a known failure mode (where the second CPU wouldn't get its IDC
interrupts set up correctly if spawned at runtime).

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