Fix unit tests certificates to have correct chains and private keys

In preparation for certificate integrity-checking: invalid certificate
chains and mismatching private keys will no longer be allowed anywhere
in Lemur code.

The test vector certs were generated using the Lemur "cryptography"
authority plugin.

* Certificates are now more similar to real-world usage: long serial
  numbers, etc.
* Private key is included for all certs, so it's easy to re-generate
  anything if needed.

lemur/plugins/lemur_slack/tests/test_slack.py

@@ -16,7 +16,7 @@ def test_formatting(certificate):
             },
             {
                 'short': True,
-                'value': u'Wednesday, January 1, 2020',
+                'value': u'Tuesday, December 31, 2047',
                 'title': 'Expires'
             }, {
                 'short': True,