LightEditorTest Fix

[ericmehl]

I think based on the improvement in the tests noted in #5891 (comment), these other tests should be checking for the absence of X instead of P.X (which will never be present)?

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]

Good catch! Thanks!