Adds a test to ensure that we clear the heap between calls into runtime (#4903)

* Adds a test to ensure that we clear the heap between calls into runtime

The tests shows that we currently not clearing the heap in wasmtime.
For now we don't run the test for wasmtime.

* Fix compilation

substrate/client/executor/wasmi/src/lib.rs

@@ -669,6 +669,19 @@ impl WasmRuntime for WasmiRuntime {
 			&self.missing_functions,
 		)
 	}
+
+	fn get_global_val(&self, name: &str) -> Result<Option<sp_wasm_interface::Value>, Error> {
+		match self.instance.export_by_name(name) {
+			Some(global) => Ok(Some(
+				global
+					 .as_global()
+					 .ok_or_else(|| format!("`{}` is not a global", name))?
+					 .get()
+					 .into()
+			)),
+			None => Ok(None),
+		}
+	}
 }
 
 pub fn create_instance(