Bugs in PHP 8.2/Increase test coverage

[RobjS]

A number of classes (e.g. the generators) have no test coverage. As a result, there are some commands that will throw errors in PHP 8.2 (e.g. whippet generate theme) as the errors were not surfaced by the test suite when we did the initial PHP 8.2 work. We should ensure we have test coverage (at least unit, if not integration) for all the codebase, to avoid this in future.

[snim2]

This might also be an opportunity to remove some repeated code in the generators and extract it into a superclass

[snim2]

Related:

• Tests should mock the filesystem consistently #226
• Refactor WhippetHelpers #227