test: improve error reporting in test-child-process-pipe-dataflow

When the test fails, it usually ends up failing on the assertion that
`wc` did not find as many bytes as it expected. That may not be
helpful if it is caused by a failure earlier in the command
pipeline ( cat | grep | wc ). Move the byte check to
`process.on('exit')` so that other error handlers that report the
existence of errors run first.

Refs: nodejs#25988

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

@@ -66,8 +66,8 @@ const MB = KB * KB;
     wcBuf += data;
   }));
 
-  wc.on('close', common.mustCall(() => {
+  process.on('exit', () => {
     // Grep always adds one extra byte at the end.
     assert.strictEqual(wcBuf.trim(), (MB + 1).toString());
-  }));
+  });
 }