💚 zv: Higher recursion limit for test builds

We've a testcase that checks if ser/de of containers exceeding the
recursion limit, error out. Since these limits may not match that of the
default rustc limits, we can end up with rustc bailing on us[1].

Let's explicitly set a high enough Rust recursion limit for zvariant crate
when we're building for tests.

zvariant/src/lib.rs

@@ -10,6 +10,7 @@
     // W/o this, we seem to get some bogus warning about `extern crate zbus`.
     allow(unused_extern_crates),
 )))]
+#![cfg_attr(test, recursion_limit = "256")]
 
 #[macro_use]
 mod utils;