feat(iota-indexer): Introduce ability to create more shared test clusters

[tomxey]

Description of change

Introduce ability to create more shared test clusters, in a similar way that more simulacrum clusters can be created in #3018 . This PR is currently stacked on top of #3018 until it's merged.

Links to any relevant issues

fixes #3492

Type of change

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

cargo test -j2 --profile simulator --features shared_test_runtime --test rpc-tests

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes