Code redundancy between ext implementation and testing. (#3830)

* fix child_storage_hash

* extract common implementation for ext and testing

* cleaning impl.

* replace ExtBasisMut by actual Ext

* remove extbasis.

* Update tests to use Ext from test externalities.

* use Ext constructor for getting ext from TestExternalities.

* Add missing extensions from ext.

* fix wasmi test

* Fix merge error.

substrate/core/executor/src/lib.rs

@@ -98,6 +98,7 @@ mod tests {
 	#[test]
 	fn call_in_interpreted_wasm_works() {
 		let mut ext = TestExternalities::default();
+		let mut ext = ext.ext();
 		let res = call_in_wasm(
 			"test_empty_return",
 			&[],