tests: msgq: run in user mode when possible

Unnecessary k_thread_abort() removed from test_msgq_purge_when_put.

A single global msgq object is now shared instead of being declared
on thread stacks, except for an ISR test case which has had its
semaphore renamed.

Moved k_sem_init() call from msgq_thread() to test_msgq_thread()
to fix a race condition.

userspace tag added to testcase.yaml.

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