GH-125866: Improve tests for `pathname2url()` and `url2pathname()` (#125993)

Merge `URL2PathNameTests` and `PathName2URLTests` test cases (which test
only the Windows-specific implementations from `nturl2path`) into the main
`Pathname_Tests` test case for these functions.

Copy/port some test cases for `pathlib.Path.as_uri()` and `from_uri()`.