Add test cases for find_uv_bin

[zanieb]

Adds test cases to unblock

• Update find_uv_bin to locate uv in the base prefix #14181
• Refactor find_uv_bin and add a better error message #14182
• Add support for --prefix and --with installations in find_uv_bin #14184
• uv 0.8.2 fails with FileNotFoundError tox-dev/pre-commit-uv#70

We use a package with a symlink to the Python module to get a mock installation of uv without building (or packaging) the uv binary. This lets us test real patterns like uv pip install --prefix without encoding logic about where things are placed during those installs.

[konstin]

Why are we using with_filter here but let mut filters = context.filters(); in the rest of the tests?

[zanieb]

I think it's a better pattern — I intend to update the other cases later.