test async: true option in Rust codegen tests

I had meant to do this originally, but apparently forgot to actually use the
option.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

crates/rust/tests/codegen.rs

@@ -59,6 +59,7 @@ mod codegen_tests {
                         stubs,
                         export_prefix: "[async-prefix]",
                         generate_all,
+                        async: true,
                     });
 
                     #[test]