Test coverage for retrieval of git credentials from the helper

Related to https://github.com/astral-sh/uv/issues/1980#issuecomment-1969576444

Our tests are not currently isolated from the global git credential helper, which is used to retrieve credentials if not provided in the URL. We should write a utility to isolate tests from the global credential helper and add test coverage where we populate a credential helper with a password to retrieve a dependency and install it without providing the password directly.