Refactor some certificate verification tests in preparation to adding

net/data/verify_certificate_chain_unittest/generate-violates-basic-constraints-pathlen-0.py

 #!/usr/bin/python
 # Copyright (c) 2015 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 2 intermediates. The first intermediate has a basic
 constraints path length of 0, so it is a violation for it to have a subordinate
 intermediate."""
 
 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')
 
 # Intermediate with pathlen 0
 intermediate1 = common.create_intermediate_certificate('Intermediate1', root)
 intermediate1.get_extensions().set_property('basicConstraints',
                                             'critical,CA:true,pathlen:0')
 
 # Another intermediate (with the same pathlen restriction)
 intermediate2 = common.create_intermediate_certificate('Intermediate2',
                                                        intermediate1)
 intermediate2.get_extensions().set_property('basicConstraints',
                                             'critical,CA:true,pathlen:0')
 
 # Target certificate.
 target = common.create_end_entity_certificate('Target', intermediate2)
 
 chain = [target, intermediate2, intermediate1]
-trusted = [root]
+trusted = common.TrustAnchor(root, constrained=False)
 time = common.DEFAULT_TIME
 verify_result = False
 
 common.write_test_file(__doc__, chain, trusted, time, verify_result)