Since commit 4b51e4c1, we return the actual thread priority, not what we
originally stored in the thread attributes.
Windows only supports 7 thread priority levels, which we map onto the 32
required by POSIX. So, only a subset of values will be returned exactly
by by pthread_getschedparam() after pthread_setschedparam().
Adjust tests priority1, priority2 and inherit1 so they only check for
round-tripping priority values which can be exactly represented.
For CI, this needs to handle process priority class "below normal
priority" as well.
Also check that the range of priority values is at least 32, as required
by POSIX.
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>