Improve test suite for integration tests

https://github.com/astral-sh/ruff/blob/1f27d53fd576e92a7d2cd4abc1761d6ad836d85b/crates/ty/tests/cli/main.rs#L600 Would be cleaner to have something like this: ``` let case = CliTest::with_files([ ( "pyproject.toml", r#" [tool.ty.environment] python-version = "3.11" "#, ), ( "libs/utils.py", r#" def add(a: int, b: int) -> int: return a + b "#, ), ( "child/test.py", r#" from utils import add stat = add(10, 15) "#, ), ])?; ```