treewide: silence mock structs unused warning

With a new version of Rust toolchain, our never constructed structs used
for testing macros began to trigger rustc warnings. They are silenced
explicitly using #[allow(unused)].

Author: wprzytula

scylla-cql/src/types/serialize/row.rs

@@ -1141,6 +1141,7 @@ mod tests {
     // Do not remove. It's not used in tests but we keep it here to check that
     // we properly ignore warnings about unused variables, unnecessary `mut`s
     // etc. that usually pop up when generating code for empty structs.
+    #[allow(unused)]
     #[derive(SerializeRow)]
     #[scylla(crate = crate)]
     struct TestRowWithNoColumns {}

scylla-cql/src/types/serialize/value.rs

@@ -2038,6 +2038,7 @@ mod tests {
     // Do not remove. It's not used in tests but we keep it here to check that
     // we properly ignore warnings about unused variables, unnecessary `mut`s
     // etc. that usually pop up when generating code for empty structs.
+    #[allow(unused)]
     #[derive(SerializeCql)]
     #[scylla(crate = crate)]
     struct TestUdtWithNoFields {}
@@ -2596,6 +2597,7 @@ mod tests {
         assert_eq!(reference, udt);
     }
 
+    #[allow(unused)]
     #[derive(SerializeCql, Debug)]
     #[scylla(crate = crate, flavor = "enforce_order", skip_name_checks)]
     struct TestUdtWithSkippedNameChecks {

scylla/tests/integration/hygiene.rs

@@ -64,6 +64,7 @@ macro_rules! test_crate {
             assert_eq!(sv, sv2);
         }
 
+        #[allow(unused)]
         #[derive(_scylla::macros::SerializeCql, _scylla::macros::SerializeRow)]
         #[scylla(crate = _scylla)]
         struct TestStructNew {