use path.relative inside relativeId, to try and fix tests on windows

src/utils/relativeId.js

@@ -1,4 +1,6 @@
+import { isAbsolute, relative } from './path.js';
+
 export default function relativeId ( id ) {
-	if ( typeof process === 'undefined' ) return id;
-	return id.replace( process.cwd(), '' ).replace( /^[\/\\]/, '' );
+	if ( typeof process === 'undefined' || !isAbsolute( id ) ) return id;
+	return relative( process.cwd(), id );
 }