Fix failing local unit tests

It appears that following the merge into main of PR #1484 that some unit tests are now failing locally.

The tests all seem to be failing because the return link URL in some of the bill run review tests now appears to be incorrect when this feature flag is set to `true` locally `ENABLE_SYSTEM_RETURNS_VIEW`.

It looks like some tests just need this feature flag stubbing in them and some extra tests added to check the URL with the flag set to both `true` and `false`.