[core] Add missing unwrap export to test-utils type definition (#12184)

packages/material-ui/src/test-utils/index.d.ts

@@ -2,3 +2,4 @@ export { default as createShallow } from './createShallow';
 export { default as createMount } from './createMount';
 export { default as createRender } from './createRender';
 export { default as getClasses } from './getClasses';
+export { default as unwrap } from './unwrap';