posix: SCHED_RR valid thread priorities

[ambroise-arm]

Describe the bug
Zephyr does not correctly advertize the minimum acceptable priority value that can be used with its posix threads when using SCHED_RR.

Calling sched_get_priority_min returns 0:

zephyr/lib/posix/pthread_sched.c

Line 39 in d5f83d2

return 0;

But looking at posix_to_zephyr_priority (used by pthread_create), 0 is not an acceptable value as it translates to a Zephyr thread priority of CONFIG_NUM_PREEMPT_PRIORITIES:

zephyr/lib/posix/pthread.c

Line 92 in d5f83d2

prio = (CONFIG_NUM_PREEMPT_PRIORITIES - priority);

which fails on the following assert when using the Zephyr test framework:

zephyr/kernel/thread.c

Line 536 in d5f83d2

Z_ASSERT_VALID_PRIO(prio, entry);

Note that the priority set by pthread_attr_init is 1, so it isn't an issue when leaving the default thread priority. But this value looks arbitrary:

zephyr/lib/posix/pthread.c

Line 21 in d5f83d2

#define LOWEST_POSIX_THREAD_PRIORITY 1

To Reproduce
Steps to reproduce the behavior:

1. Use CONFIG_PREEMPT_ENABLED=y in order to get SCHED_RR.
2. Use CONFIG_ZTEST=y in order to get the kernel asserts.
3. Have something like the following:

pthread_attr_t attr;
struct sched_param sched_param;
sched_param.sched_priority = sched_get_priority_min();
pthread_attr_init(&attr);
pthread_attr_setschedparam(&attr, &sched_param);
pthread_create(..., &attr, ...);

Expected behavior
No assert failed when using a priority advertized by Zephyr as being valid.

Impact
Annoyance

Logs and console output
With CONFIG_NUM_PREEMPT_PRIORITIES=15:

ASSERTION FAIL [((((prio)) == 15 && z_is_idle_thread_entry((entry))) || (((15 - 1) >= ((-16))) && ((prio)) >= ((-16)) && ((prio)) <= (15 - 1)))] @ WEST_TOPDIR/zephyr/kernel/thread.c:536
	invalid priority (15); allowed range: 14 to -16

Environment (please complete the following information):
Tested on tag zephyr-v3.3.0, looks still relevant on tip of main as of the time of writing.

Additional context
N/A

cc @PatrickM-ZS