tests: semaphore: run some threads in user mode

main.c and test_sema_contexts.c merged

userspace tag added to testcase.yaml

stack-allocated semaphore in test_sema_thread2thread now just uses
the global semaphore with the same name.

ISR tests run in supervisor mode.

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