Add Subgraph Creation Tests (#1415)

## Description

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Adds a bunch of tests to cover recently introduced methods & utilities for subgraph creation:
- checkExternalInputConnections
- handleGroupNodes
- createSubgraphFromNodes

## Related Issue and Pull requests

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## Type of Change

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- [x] Improvement

## Checklist

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- [ ] I have tested this does not break current pipelines / runs functionality
- [ ] I have tested the changes on staging

## Screenshots (if applicable)

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## Test Instructions

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## Additional Comments

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Author: camielvs