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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[rcooke-warwick]

@balena-ci I self-certify!

[floion]

@rcooke-warwick I assume you meant CDS instead of CBS here? And about that, should we even mention CDS?

[floion]

@resin-jenkins test this please