Remove calls to atexit in tests

When using ``atexit.register(connect.Disconnect, si)`` in a test
the test framework process will create extra trace when it exits.
To avoid producing this trace, remove all calls to atexit in tests.

fixes: https://github.com/vmware/pyvmomi/issues/131
Merge pull request #146 from hartsock/issues/131

Remove calls to atexit in tests