cleanup: replace magic TimeSlice::new(100) and Priority::new(16) with DEFAULT

TimeSlice::DEFAULT and Priority::DEFAULT already exist with the
same values. Using them makes the intent clear and centralizes
the default values.

TimeSlice::new(2) in tick test is intentional — tests preemption
after 2 ticks specifically.

crates/kernel/src/tests/ipc.rs

@@ -29,8 +29,8 @@ fn setup_ipc_kernel() -> (
     let pid_b = ProcessId::from_raw(1);
     let tid_b = k.spawn_thread_for_test(
         pid_b,
-        Priority::new(16).unwrap(),
-        TimeSlice::new(100),
+        Priority::DEFAULT,
+        TimeSlice::DEFAULT,
     );
 
     // Create an endpoint