tests: alert_api: run in user mode

main.c and test_alert_contexts.c merged.

User threads can't look inside the alert structures, so an extra
variable 'htype' introduced to track expectations for any given
alert object in alert_recv().

Alert objects have to be initialized by supervisor threads since
they register callbacks. An array of toplevel alert objects created
and initialized in test_main(), replacing the ones that used to
live on thread stacks.

Added userspace tag to testcase.yaml

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>