Add tests for Vulnerabilities::FindingsPreloader

The following discussion from !31982 (merged) should be addressed:

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

  @.luke:

  I'm a little surprised that there was no existing test of the class, however, if the lack of N+1 is tested for somewhere else, then that would be understandable. If there's not, there may be an opportunity to add such a test as a follow-up

Technical Implementation

• backend
  • Add spec file and tests for Vulnerabilities::FindingsPreloader