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/system/benches/bench.rs

@@ -16,10 +16,7 @@
 // limitations under the License.
 
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use frame_support::{
-	traits::{ConstU32, ConstU64},
-	weights::Weight,
-};
+use frame_support::traits::{ConstU32, ConstU64};
 use sp_core::H256;
 use sp_runtime::{
 	testing::Header,