Use substring instead of the deprecated string method substr in tests.

Author: jaydenseric

changelog.md

@@ -28,6 +28,7 @@
 - Reorganized the test file structure.
 - Use the `.js` file extension in `require` paths.
 - Use the Node.js `Readable` property `readableEncoding` instead of `_readableState.encoding` in tests.
+- Use `substring` instead of the deprecated string method `substr` in tests.
 - Fixed a typo in a code comment.
 - Updated documentation.
 - Added a `license.md` MIT License file, closing [#86](https://github.com/jaydenseric/graphql-upload/issues/86).

test/abortingMultipartRequest.mjs

@@ -44,7 +44,7 @@ export default async function abortingMultipartRequest(
       if (chunkAbortIndex !== -1) {
         if (chunkAbortIndex !== 0)
           // Yield the final truncated chunk before aborting.
-          yield textEncoder.encode(chunkString.substr(0, chunkAbortIndex));
+          yield textEncoder.encode(chunkString.substring(0, chunkAbortIndex));
 
         // Abort the request after it has been received by the server request
         // handler, or else Node.js won’t run the handler.