Allow AVA helper filename convention

[jamestalmage]

I currently get the following error running the test suite for ava-files.

  fixtr/default-patterns/test/_foo-help.js:1:9
  ✖    1:9  Filename is not in kebab case. Rename it to foo-help.js  xo/filename-case

Should we allow leading underscores? Should we only allow it in the default test and fixture folders?

Note: ava-files does not use the default fixture folder (so if we only allow it for the default folders, we would need to explicitly add an exception anyways).

Ref: avajs/ava-files@3f2a63a

[sindresorhus]

I think we should allow it anywhere. It's common convention in many ecosystems.

This issue would be better opened on eslint-plugin-unicorn though.

[sindresorhus]

Moved to sindresorhus/eslint-plugin-unicorn#79.