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`.
This test and the test below are exactly the same test case since `urlparse` parses them in the exact same way:

```pycon
>>> urlparse.urlparse('localhost.localdomain:3128/')
ParseResult(scheme='localhost.localdomain', netloc='', path='3128/', params='', query='', fragment='')
>>> urlparse.urlparse('10.122.1.1:3128/')
ParseResult(scheme='10.122.1.1', netloc='', path='3128/', params='', query='', fragment='')
```

They should be consolidated.