Show tracing output on test failure

In the vein of https://github.com/astral-sh/uv/pull/8349, it's nice to have more information from the child process when a test fails. Right now, if you turn tracing on it ends up in the snapshot and breaks the test. We should have tracing on by default, but exclude it from the snapshots and display it on failure. This could be implemented with something like a UV_LOG_FILE option we pass to the child process then read from on failure.