Update unit tests to not inherit environment

This commit updates the unit tests to always start with a known set of
environment variables rather than inheriting the environment from the
shell running the tests. This avoids breakage when there's something
in the environment that AsyncSSH looks for that the test case didn't
intend to set.

tests/server.py

@@ -132,8 +132,7 @@ def asyncSetUpClass(cls):
                                                       cls._server_port))
         run('cat skey.pub >> .ssh/known_hosts')
 
-        os.environ['LOGNAME'] = 'guest'
-        os.environ['HOME'] = '.'
+        os.environ = {'LOGNAME': 'guest', 'HOME': '.'}
 
         try:
             output = run('ssh-agent -a agent 2>/dev/null')