tests/check.sh: Fix a race

Fixes this random failure:

    error: hash mismatch in fixed-output derivation '/tmp/nix-shell.EUgAVU/nix-test/tests/check/store/sfps3l3c5n7dabpx34kigxnfhmrwk2h6-dummy.drv':
             specified: sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
                got:    sha256-0qhPS4tlCTfsj3PNi+LHSt1akRumTfJ0WO2CKdqASiY=

which happens because multiple tests were writing to ./dummy.