getpayload* tests: Don't assume that '-' works as expected on NaN. * tests/test-getpayload.c: Include signed-nan.h. (main): Use minus_NaNd. * tests/test-getpayloadf.c: Include signed-nan.h. (main): Use minus_NaNf. * tests/test-getpayloadl.c: Include signed-nan.h. (main): Use minus_NaNl. * modules/getpayload-tests (Depends-on): Add signed-nan. * modules/getpayloadf-tests (Depends-on): Likewise. * modules/getpayloadl-tests (Depends-on): Likewise.