Because the `HostKeyTest` class was mutating its static/global
`HOST_KEY_TYPES` dict, this class could not actually be used more than once
in a single thread!
Rather than mutate this dict after parsing each key type
(`HOST_KEY_TYPES[host_key_type]['parsed'] = True`), the `perform_test`
method should simple add the parsed key types to a local `set()`.
