chore: silence clippy::redundant_clone warning (#2619)

The purpose of this test is to assert two clones of the same span are
equal to each other, so the clone is kind of the whole point of the
test. This commit adds an allow attribute to make clippy shut up about
it.

tracing/tests/span.rs

@@ -21,7 +21,12 @@ fn handles_to_the_same_span_are_equal() {
     // expect to see any spans.
     with_default(collector::mock().run(), || {
         let foo1 = tracing::span!(Level::TRACE, "foo");
+
+        // The purpose of this test is to assert that two clones of the same
+        // span are equal, so the clone here is kind of the whole point :)
+        #[allow(clippy::redundant_clone)]
         let foo2 = foo1.clone();
+
         // Two handles that point to the same span are equal.
         assert_eq!(foo1, foo2);
     });