ceil, floor, round, trunc: enhance tests of -0

ceil, floor, round, trunc: enhance tests of -0

* tests/test-ceilf1.c (main): Ensure correct sign of result.
* tests/test-ceill.c (main): Likewise.
* tests/test-floorf1.c (main): Likewise.
* tests/test-floorl.c (main): Likewise.
* tests/test-round1.c (main): Likewise.
* tests/test-roundf1.c (main): Likewise.
* tests/test-roundl.c (main): Likewise.
* tests/test-trunc1.c (main): Likewise.
* tests/test-truncf1.c (main): Likewise.
* tests/test-truncl.c (main): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>