Bug 23843: Make funds endpoint tests more robust

Bug 23843: Make funds endpoint tests more robust

Now that we use ->to_api all the object attributes are returned, so the
original tests that didn't follow the FKs would pass, and now they
don't. If there is no budget linked to the fund, it will return undef,
which is not accepted on the API (though it accepts not returning the
attribute).

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/acquisitions_funds.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>