Add prepend_mod to BillingPlansHelper and skip tests for JH

As title, Change module BillingPlansHelper to prependable

And skip the test for now. Since this cause related tests broken in jh pipeline

See MR of JH gitlab-jh/gitlab!59 (merged)

we can only skip it for temp and fix it in the future.

There is a precedent https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/subscription_portal_spec.rb#L5

Please help review this, thanks. @godfat-gitlab CC: @qianzhangxa