Add tests to test_invalid_url for `InvalidSchema`

In b149be5d `PreparedRequest` was made to skip `parse_url` for e.g.
`$HOST:$PORT`, which results in `MissingSchema` not being raised for
`localhost:3128`.
What I've meant is that this tests the skipping code: if this was changed, e.g. by using a more sophisticated regex, the behavior might change and this additional test might catch it.