Cycles : Correct setting of background light's lightgroup

[boberfly]

Generally describe what this PR will do, and why it is needed

• The lightgroup name wasn't being set on background lights properly
• Added the lightgroup option on CyclesOptions for if CyclesBackground is being used instead of a background light
• From what Brecht has said on their roadmap, they will allow more than one background light and lightgroup name each, so we should re-visit this for when that happens
• Added a unit-test to verify that the lightgroup is being picked up

Related issues

• NA

Dependencies

• NA

Breaking changes

• NA

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Closing in favour of #5945. I think it's reasonable to omit the cycles:background:lightgroup option as that PR does, with the guidance being "if you need a light group, use a light". Feel free to reopen the discussion if you think there's a good case otherwise...