Remove all mentions of CxxTest

[mere-human]

There are mentions of CxxTest as a dependency: https://github.com/tfussell/xlnt/blob/master/CONTRIBUTING.md#pull-request-checklist https://github.com/tfussell/xlnt/blob/master/LICENSE.md#cxxtest https://github.com/tfussell/xlnt/blob/master/docs/xlnt.3#L1466

However, it seems the project has been migrated to a different framework: https://github.com/tfussell/xlnt/commit/835e36d6ae717cd4bed5f6252657fa69e2378749 There is no mentions of CxxTest in the source code.

Should the CONTRIBUTING and LICENSE be updated?