Add test cases for find_uv_bin (#15110)

Adds test cases to unblock

- https://github.com/astral-sh/uv/pull/14181
- https://github.com/astral-sh/uv/pull/14182
- https://github.com/astral-sh/uv/pull/14184
- https://github.com/astral-sh/uv/pull/14184
- https://github.com/tox-dev/pre-commit-uv/issues/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.

---------

Co-authored-by: konstin <konstin@mailbox.org>

scripts/packages/fake-uv/README.md

@@ -0,0 +1,3 @@
+This fake uv package symlinks the Python module of uv in-tree and has a fake `uv` binary, allowing
+testing of the Python module behaviors. Consumers can replace the `uv` binary with a debug binary or
+similar if they need it to actually work.

scripts/packages/fake-uv/pyproject.toml

@@ -0,0 +1,11 @@
+[project]
+name = "uv"
+version = "0.1.0"
+requires-python = ">=3.8"
+
+[tool.uv.build-backend.data]
+scripts = "scripts"
+
+[build-system]
+requires = ["uv_build>=0.8.0,<0.9"]
+build-backend = "uv_build"

scripts/packages/fake-uv/scripts/uv

@@ -0,0 +1,4 @@
+#!/usr/bin/env sh
+
+echo "This is a fake uv binary"
+

scripts/packages/fake-uv/scripts/uv.exe

@@ -0,0 +1 @@
+This is a fake uv binary

scripts/packages/fake-uv/src

@@ -0,0 +1 @@
+../../../python/