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

* Only load KZG once if necessary  in tests.

crypto/kzg/src/lib.rs

@@ -142,3 +142,11 @@ impl Kzg {
         .map_err(Into::into)
     }
 }
+
+impl TryFrom<TrustedSetup> for Kzg {
+    type Error = Error;
+
+    fn try_from(trusted_setup: TrustedSetup) -> Result<Self, Self::Error> {
+        Kzg::new_from_trusted_setup(trusted_setup)
+    }
+}