IntegrityTest implementation should be feature gated (#13094)

* `IntegrityTest` implementation should be feature gated

The initial implementation for the old declarative macros is still feature gating the
implementation. As we only call this in a test, there is no need to have this compiled for wasm.

* Don't assume that all "consumers" have a `std` feature

substrate/frame/support/src/lib.rs

@@ -2744,3 +2744,6 @@ pub mod pallet_macros {
 		type_value, unbounded, validate_unsigned, weight, whitelist_storage,
 	};
 }
+
+// Generate a macro that will enable/disable code based on `std` feature being active.
+sp_core::generate_feature_enabled_macro!(std_enabled, feature = "std", $);