docs: Elaborate automated testing requirement in board support guide

[rcooke-warwick]

[skip ci]

Change-type: patch

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Contributor checklist

• [ ] Changes have been tested
  • [ ] Covered in automated test suite
  • [ ] Manual test case recorded
• [ ] Change-type present on at least one commit
• [ ] Signed-off-by is present
• [ ] The PR complies with the Open Embedded Commit Patch Message Guidelines

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  • 'Approve' if this change would be acceptable in the codebase (even if there are minor or cosmetic tweaks that could be improved).
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[rcooke-warwick]

@balena-ci I self-certify!

[floion]

@resin-jenkins test this please