tests: lib: ringbuffer: Fix preempt_cnt not being reset

Counter was not reset between various tests which may hide the fact
that number of preemptions is less than expected.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

tests/lib/ringbuffer/src/concurrent.c

@@ -343,6 +343,7 @@ static void thread_entry_spsc(void *p1, void *p2, void *p3)
 		     NULL);
 	k_timer_user_data_set(&timer, rbuf);
 
+	preempt_cnt = 0;
 	consume_fn(rbuf, true);
 	produce_fn(rbuf, true);