test: fix child-process-pipe-dataflow

Make sure all the `wc` process stdout data is received before checking
its validity.

Fixes: #25988

PR-URL: #36366
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-pipe-dataflow.js

@@ -61,8 +61,13 @@ const MB = KB * KB;
     }));
   });
 
+  let wcBuf = '';
   wc.stdout.on('data', common.mustCall((data) => {
+    wcBuf += data;
+  }));
+
+  wc.on('close', common.mustCall(() => {
     // Grep always adds one extra byte at the end.
-    assert.strictEqual(data.toString().trim(), (MB + 1).toString());
+    assert.strictEqual(wcBuf.trim(), (MB + 1).toString());
   }));
 }