[core] use hex instead of binary for file name in test (#29157)
[core] use hex instead of binary for file name in test (#29157)

Address test flakiness

The random file name may contain weird character as a filename for the filesystem. Use only alphanumeric characters instead.

Example BK failure & error:

https://buildkite.com/ray-project/oss-ci-build-branch/builds/401#0183b0c0-11fd-410a-9b16-45442842e1fb
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray