[core] Deflakey gcs server rpc test

[fishbone]

Why are these changes needed?

The test failed because in the current ray redis client there is a global variable using io context. and the io context is freed during destruction and when destruct the global variable it'll cause issues.

The current redis client is aweful and ugly. Since we'll move to redis plus plus, and this client doesn't cause other issues, I'll just fix the test case.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[rickyyx]

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