Fix all the tests

This PR fixes all the tests so that running `./setup.py test` runs successfully. The biggest change I've done is added a `Server.close()` method, as the tests weren't making sure that the `IdentDB` was getting closed, which caused an occasional race condition on the database file. This caused the whole diff to be messy, so I've split out the other fixes from it. The biggest challenge is that a number of tests referenced `pki/mykey.pem` and `pki/mycert.pem`, which were not in the repository. I've changed them to point at equivalent files that have already been checked in. There were also a number of files that were using relative rather than absolute paths.