tests: kernel/smp_boot_delay: workaround occassionally failures

test_smp_boot_delay sometimes fails due to thread started by IPI
not having started or not finished running:

* Using CPU mask to explicitly state which CPU to start the thread
  seems to fix the issue where the thread is not started quickly
  enough.
* When the host  system is under heavy load (e.g. twister-ing),
  emulators may not get enough CPU time to run the newly created
  thread. So extend the IPI delay a bit more to allow for this.

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