Add tests for unintentional filtering bug in MR

!3872 (merged) has a lack of tests for Merge Requests while !3872 (merged) has only ones for Issues.

This MR has complementary tests for MR list.

@grzesiek could you have time to review this test code related to !3872 (merged)?

@tnir Thanks! Looks good /cc @rymai

mentioned in commit 1c86e804

Status changed to merged

Thanks @tnir!