Add specs for Integration#boolean_accessor and API::Entities::ProjectIntegration, API::Entities::ProjectIntegrationBasic

The following discussion from !64185 (merged) should be addressed:

• @toupeira started a discussion: (+2 comments)

  suggestion (non-blocking): We should add unit tests for this, but we currently don't have any for Integration.boolean_accessor so I propose we add them in a follow-up MR.

The following discussion from !64185 (merged) should be addressed:

• @toupeira started a discussion:

  suggestion (non-blocking): We should add unit tests for this as well, but again I would propose we do this in a follow-up MR.