Reorganise tests in 'test_url.py'

[tomchristie]

I would like to deal with #2883.

In order to tackle this comprehensively I'm suggesting we start by tidying up our existing URL test cases.
There are two parts to this:

• Reorganise the existing tests in models/test_url.py - The current ordering of the tests isn't coherent.
• Remove test_urlparse.py, moving test cases into models/test_url.py - All the test cases there are now properly against the public httpx.URL() interface, tackled previously as part of the Refactor test cases that import from private namespace. #2492.

This pull request deals with the first of these two bullet-points.
I'll follow up with the second part once this is in.

Here's the explanation of the proposed change (assuming I've got it correct 😅)...

• Tests are re-ordered, these no removal, addition, or modifying of test cases.
• Some docstrings have been added.
• Some comments helping claify the structure have been added.

The ordering for the test cases here is...

• General tests for httpx.URL.
• General tests for httpx.QueryParams.
• Tests for httpx.URL.join().
• Tests for httpx.URL.copy_with().
• Tests for httpx.URL.copy_<...>.params() methods.
• Tests for IDNA hostname support.
• Tests for IPv6 hostname support.

[tomchristie]

Thanks @Kludex, @michaeloliverx. 🙏🏼