v15: Add tests for ksm_metadata_v15.bin

[lexnv]

Once the metadata V15 is deployed on the Kusama chain, add the metadata v15 binary to this repo.
The V15 metadata is then used to add a test that ensures metadata V15 can be decoded properly.

	#[test]
	fn should_decode_metadatav15() {
		let meta: RuntimeMetadataPrefixed =
			Decode::decode(&mut load_metadata(15).as_slice()).unwrap();
		assert!(matches!(meta.1, RuntimeMetadata::V15(_)));
	}

Depends on: paritytech/substrate#13302.