Correct usage of invalid file URL in test

[freakboy3742]

In the process of diagnosing test failures in #12962, I discovered that there has been a change in Python 3.13.0rc2 (that wasn't in rc1) that breaks the pip test suite.

The underlying cause is python/cpython#85110, which has been back ported to 3.13 and 3.12. Previously, the following code:

>>> from urllib.parse import urlunsplit
>>> urlunsplit(('file', '', 'nonexistent', '', ''))
'file:///nonexistent'

whereas it now returns:

file:nonexistent

This poses a problem because the absence of the file:// means that the appropriate URL handler isn't installed, resulting in a failure of "ERROR: Could not install packages due to an OSError: No connection adapters were found for 'file:unexistent_file'" rather than "ERROR: 404 Client Error: FileNotFoundError for url: file:///unexistent_file".

The new behavior seems correct - file:unexistent_file isn't a valid File URL; and it definitely isn't equivalent to file:///unexistent_file (at best, it would be equivalent to file://unexistent_file).

This PR updates the test case to use a valid, but non-existent file URL (which seems to be the point of the test).

[freakboy3742]

@uranusjr Just to confirm I haven't missed something here - is there anything pending on me for this PR?