Fix: Add `tests/helpers` to `npm` package
Fix: Add `tests/helpers` to `npm` package

This will enable developers to use the same infrastructure
we do to test their custom rules.

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Ref https://github.com/sonarwhal/sonar/issues/528#issuecomment-331321328
Close #532

sonarwhal/sonar,alrra/sonar,alrra/sonar,sonarwhal/sonar,alrra/sonar,sonarwhal/sonar