sim: Implement Default for a few types

Clippy suggests implementing Default when the `new` function for that
type just initializes the type to default values.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index e602327..6cea000 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -169,6 +169,7 @@
     }
 }
 
+#[derive(Default)]
 pub struct RunStatus {
     failures: usize,
     passes: usize,