doc/go1.17: mention testing.[TB].Setenv methods

For #41260
For #44513

Change-Id: I47ac0c751dafeb05abfe66fdf77938774164915f
Reviewed-on: https://go-review.googlesource.com/c/go/+/323353
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: roger peppe <[email protected]>

Author: ianlancetaylor

doc/go1.17.html

@@ -628,6 +628,14 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     <p><!-- CL 310033 -->
       TODO: <a href="https://golang.org/cl/310033">https://golang.org/cl/310033</a>: add -shuffle=off|on|N to alter the execution order of tests and benchmarks
     </p>
+
+    <p><!-- CL 260577 -->
+      The new
+      <a href="/pkg/testing/#T.Setenv"><code>T.Setenv</code></a>
+      and <a href="/pkg/testing/#B.Setenv"><code>B.Setenv</code></a>
+      methods support setting an environment variable for the duration
+      of the test or benchmark.
+    </p>
   </dd>
 </dl><!-- testing -->