Retry failed unit tests using dune cache

[mrmr1993]

This PR tweaks the unit test runner script, so that it will retry the tests once within the same buildkite run. This allows any flaking test to be re-run without a rebuild, and without running the other tests that have already succeeded.

If the test still fails, either due to a badly configured/misbehaving buildkite node or a genuine error in the PR, then the normal buildkite retry process will still take effect.

Checklist:

• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them