Add specs for multiple assignees issue board cards

What does this MR do?

Add specs for the changes made in !1703 (merged)

Are there points in the code the reviewer needs to double check?

Shouldn't be. Karma will fail if you run all the tests (because this branch contains multiple assignees changes that have yet to had their specs fixed) but my additions should pass.

Why was this MR needed?

Adds tests

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

Yes

What are the relevant issue numbers?

Closes #2258 (closed)