testing: fix ResourceWarning in broken-pipe test

Author: bluetech

testing/acceptance_test.py

@@ -1288,3 +1288,6 @@ def test_no_brokenpipeerror_message(pytester: Pytester) -> None:
     ret = popen.wait()
     assert popen.stderr.read() == b""
     assert ret == 1
+
+    # Cleanup.
+    popen.stderr.close()