test: consolidate and prune redundant unit tests

Simplify the test suite by merging similar test cases and removing redundant assertions that verify trivial initialization and basic container logic.

crates/irtt-client/src/managed/cancellation.rs

@@ -27,11 +27,6 @@ impl CancellationToken {
 mod tests {
     use super::*;
 
-    #[test]
-    fn token_starts_uncancelled() {
-        assert!(!CancellationToken::new().is_cancelled());
-    }
-
     #[test]
     fn cancel_is_idempotent_and_visible_through_clones() {
         let token = CancellationToken::new();