chore: replace deprecated String.prototype.substr()

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() which works similarily but isn't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

test/helpers/readAsset.js

@@ -10,7 +10,7 @@ export default (asset, compiler, stats) => {
   const queryStringIdx = targetFile.indexOf("?");
 
   if (queryStringIdx >= 0) {
-    targetFile = targetFile.substr(0, queryStringIdx);
+    targetFile = targetFile.slice(0, queryStringIdx);
   }
 
   try {