tests: use "returns_" rather than explicit comparison with "$?"

tests: use "returns_" rather than explicit comparison with "$?"

* tests/chmod/no-x.sh: Likewise.
* tests/cp/no-deref-link1.sh: Likewise.
* tests/cp/no-deref-link2.sh: Likewise.
* tests/cp/no-deref-link3.sh: Likewise.
* tests/du/move-dir-while-traversing.sh: Likewise.
* tests/ls/infloop.sh: Likewise.
* tests/ls/stat-failed.sh: Likewise.
* tests/ls/time-style-diag.sh: Likewise.
* tests/misc/chroot-fail.sh: Likewise.
* tests/misc/env-null.sh: Likewise.
* tests/misc/env.sh: Likewise.
* tests/misc/nice-fail.sh: Likewise.
* tests/misc/printenv.sh: Likewise.
* tests/misc/timeout.sh: Likewise.
* tests/tail-2/follow-name.sh: Likewise.