Put the new tests in their own nested mod

The module is in the same file, nested inside the `tests` module
written there (which this test code had resided directly inside).
It is currently named `loc`.

This allows the repeated `#[cfg(windows)]` to be eliminated, allows
the imports to be presented readably and to be grouped without
`cargo fmt` undoing the grouping, and makes clear that these tests
are all related to each other more than to the preexsting tests.

But I am not sure if this is the best approach to organizing them.
Maybe the test code for `locations_under_program_files`, which is a
helper function meant to be used only in this module and only to
define `ALTERNATIVE_LOCATIONS`, could remain in this file, while
the other tests of `ALTERNATIVE_LOCATIONS` could be moved to
another file. `ALTERNATIVE_LOCATIONS` has `pub(super)` visibility,
so it's not part of the interface of `gix_path::env`, but it could
perhaps have tests in another file directly or indirectly inside
`gix-path/src/env`.

Author: EliahKagan