Refactor some certificate verification tests in preparation to adding

net/data/verify_certificate_chain_unittest/generate-expired-root.py

 #!/usr/bin/python
 # Copyright (c) 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Certificate chain with 1 intermediate, where the root certificate is expired
 (violates validity.notAfter). Verification is expected to succeed as
 constraints on trust anchors are not enforced.."""
 
 import common
 
-# Self-signed root certificate (part of trust store).
+# Self-signed root certificate (used as trust anchor).
 root = common.create_self_signed_root_certificate('Root')
 root.set_validity_range(common.JANUARY_1_2015_UTC, common.MARCH_1_2015_UTC)
 
 # Intermediate certificate.
 intermediate = common.create_intermediate_certificate('Intermediate', root)
 intermediate.set_validity_range(common.JANUARY_1_2015_UTC,
                                 common.JANUARY_1_2016_UTC)
 
 # Target certificate.
 target = common.create_end_entity_certificate('Target', intermediate)
 target.set_validity_range(common.JANUARY_1_2015_UTC, common.JANUARY_1_2016_UTC)
 
 chain = [target, intermediate]
-trusted = [root]
+trusted = common.TrustAnchor(root, constrained=False)
 
 # Both the target and intermediate are valid at this time, however the
 # root is not. This doesn't matter since the root certificate is
 # just a delivery mechanism for the name + SPKI.
 time = common.MARCH_2_2015_UTC
 verify_result = True
 
 common.write_test_file(__doc__, chain, trusted, time, verify_result)