Add test for `BuildCoupled_SWISS`

[danielhollas]

We now have an almost perfect test coverage of SelectionModule, the only code path not tested is this branch in BuildCoupled_SWISS.

This can be triggered only when two TBFs have the same parent but different selection time.

[danielhollas]

@yoricklassmann I assigned you, would be great to have but obviously no rush at all. I am just copying over issues from the fms90-redux repo