findutils: Disable tests on Darwin

The strftime test fails with:

NZST-12NZDT,M9.5.0,M4.1.0/3: expected "1970-01-01 13:00:00 +1300 (NZDT)", got "1970-01-01 12:00:00 +1200 (NZST)"

The source in fact marks this test as "ahistorical", so the result on
Darwin might not be incorrect.

http://hydra.nixos.org/build/32578562/nixlog/1/raw