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/helpers.rs

@@ -23,7 +23,7 @@ pub fn test_kernel() -> Kernel<MockPlatform> {
 pub fn kernel_with_process() -> (Kernel<MockPlatform>, ProcessId) {
     let mut kernel = test_kernel();
     let (pid, _tid) = kernel
-        .boot(Priority::new(16).unwrap(), TimeSlice::new(100))
+        .boot(Priority::DEFAULT, TimeSlice::DEFAULT)
         .unwrap();
     (kernel, pid)
 }