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`.
They are still functionally equivalent. They're not an implementation detail because as I mentioned, RFC 3986 has no way of identifying that the part before the `:` here is not a scheme. So any specification compliant implementation will do this, ergo it's a specification detail that makes these tests functionally equivalent.