Add test coverage for custom helpers

[khiga8]

This PR:

• adds missing test coverage for the logic we have to allow linters to be disabled in a file
• adds missing test coverage for the custom helpers we use.
• updates the logic that allows linter rules to be a bit more conflict proof.

[khiga8]

Our linters have logic that allows a linter to be disabled/ignored in a file. This comes from: rule_disabled?

Right now, we only check the demodulized name so:

<%# erblint:disable NoRedundantImageAlt %>

and a totally invalid name like:

<%# erblint:disable BadModule::NoRedundantImageAlt %>

can be used to disable a linter rule in a file.

I updated this line, to require the fuller linter name (not including ERBLint::Linters bcuz that is redundant).

Our linter names are namespaced and this seems more conflict proof!

[joelhawksley]

It might be time to add a CHANGELOG 😄