tests: add bridge networking test

Add test_multiple_containers_same_internal_port() to verify the key
benefit of bridge networking: multiple containers can listen on the
same internal port with different host port mappings.

The test runs two nginx containers both listening on port 80 internally,
mapped to host ports 8080 and 8081, and verifies both are accessible.

Also update TestPortForwarding docstring to reflect the change from
host networking to bridge networking.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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