tests: cmsis_rtos_v2: wait a bit for thread1 to set flags

In test_event_flags_no_wait_timeout(), after creating thread1,
there is a little delay to let it run to trigger FLAG1.
However, in test_event_flags_signalled(), this is not being
done, and on some platforms it triggers the assert in thread1()
complaining the flag not being set. This adds the same delay
in test_event_flags_signalled() and the this test passes for
those previously failed platforms.

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