doc/go1.15: add line for testing streaming change

Updates #37419.
Updates #38458.
Updates #24929.

Change-Id: I793bb20fa9db4432fc3a5b69956b7108e4695081
Reviewed-on: https://go-review.googlesource.com/c/go/+/241660
Run-TryBot: Jean de Klerk <deklerk@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.15.html

@@ -956,7 +956,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     </p>
 
     <p><!-- CL 229085 -->
-      TODO: <a href="https://golang.org/cl/229085">https://golang.org/cl/229085</a>: reformat test chatty output
+      <code>go</code> <code>test</code> <code>-v</code> now groups output by
+      test name, rather than printing the test name on each line.
     </p>
   </dd>
 </dl><!-- testing -->