https://github.com/pexpect/pexpect/pull/527
commit 5349569bcad4ab1685c9f2ec2a2c77a69cde4e48
Author: Bernhard M. Wiedemann <bwiedemann@suse.de>
Date:   Tue Sep 4 13:57:43 2018 +0200

    tests: ignore stderr from openssl
    
    Without this patch, the test failed with openssl-1.0.2p
    because 502 lines were captured with the extra lines being at the end:
    140510355240592:error:02012020:system library:fflush:Broken pipe:bss_file.c:434:fflush()
    140510355240592:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:436:
    
    neither openssl-1.0.2j nor 1.1.0h needed this patch.

diff --git a/tests/test_expect.py b/tests/test_expect.py
index 795518a..2c74744 100755
--- a/tests/test_expect.py
+++ b/tests/test_expect.py
@@ -411,7 +411,7 @@ class ExpectTestCase (PexpectTestCase.PexpectTestCase):
     def test_before_across_chunks(self):
         # https://github.com/pexpect/pexpect/issues/478
         child = pexpect.spawn(
-            '''/bin/bash -c "openssl rand -base64 {} | head -500 | nl --number-format=rz --number-width=5 2>&1 ; echo 'PATTERN!!!'"'''.format(1024 * 1024 * 2),
+            '''/bin/bash -c "openssl rand -base64 {} 2>/dev/null | head -500 | nl --number-format=rz --number-width=5 2>&1 ; echo 'PATTERN!!!'"'''.format(1024 * 1024 * 2),
             searchwindowsize=128
         )
         child.expect(['PATTERN'])