Make "cargo miri test" actually run

src/machine/arithmetic_ops.rs

@@ -1423,6 +1423,7 @@ mod tests {
     use crate::machine::mock_wam::*;
 
     #[test]
+    #[cfg_attr(miri, ignore = "blocked on streams.rs UB")]
     fn arith_eval_by_metacall_tests() {
         let mut wam = MachineState::new();
         let mut op_dir = default_op_dir();