The test cases where failing because the test
`test_converts_Trix-formatted_attachments_with_custom_tag_name` set a
custom tag_name to `arbitrary-tag`. This test would also set the
ActionText::AttachmentGallery::ATTACHMENT_SELECTOR private constant
to `arbitrary-tag`.

Other test cases had proper ActionText::Attachment.tag_name set to
`action-text-attachment` but the constant once defined would not
reset.

This PR attempts to fix the issue by converting the
ActionText::AttachmentGallery::{ATTACHMENT_}SELECTOR to class methods

Fixes #41782