Only load Kzg in tests if necessary and only load it once (#5555)

* Only load KZG once if necessary  in tests.

beacon_node/execution_layer/src/test_utils/mod.rs

@@ -109,7 +109,7 @@ impl<E: EthSpec> MockServer<E> {
     pub fn new_with_config(
         handle: &runtime::Handle,
         config: MockExecutionConfig,
-        kzg: Option<Kzg>,
+        kzg: Option<Arc<Kzg>>,
     ) -> Self {
         let MockExecutionConfig {
             jwt_key,
@@ -193,7 +193,7 @@ impl<E: EthSpec> MockServer<E> {
         shanghai_time: Option<u64>,
         cancun_time: Option<u64>,
         prague_time: Option<u64>,
-        kzg: Option<Kzg>,
+        kzg: Option<Arc<Kzg>>,
     ) -> Self {
         Self::new_with_config(
             handle,