GafferCycles : Custom AOV fix + unit test

[boberfly]

https://groups.google.com/g/gaffer-dev/c/m8NXKI9NIVE

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

• Fix for incorrectly passing the wrong pass name enum when constructing a custom AOV CyclesOutput, which caused custom AOVs to never be rendered unless they were named aov_color or aov_value
• Unit test added for custom AOVs

Related issues

Dependencies

Breaking changes

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]

Thanks Alex, it's great to see the test coverage continue to ramp up like this! I've made a comment inline about standardising on float/color syntax, which - unless I missed something - I think is probably worth doing while we're still reserving the right to break things to improve things in GafferCycles.