test(tvix/serde): add initial set of deserialisation tests

Change-Id: I0a9779edf0296c25d37fb5f75f8fc5852fe00121
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7715
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>

tvix/serde/src/lib.rs

@@ -6,3 +6,6 @@ mod de;
 mod error;
 
 pub use de::from_str;
+
+#[cfg(test)]
+mod de_tests;