tests: improve diagnostics when asserting empty files

tests: improve diagnostics when asserting empty files

* tests/chmod/c-option.sh: Use `compare /dev/null ... || fail=1`
rather than `test -s ... && fail=1`, so that the file contents
are output, thus improving diagnostics for failing tests.
* tests/cp/acl.sh: Likewise.
* tests/cp/cp-a-selinux.sh: Likewise.
* tests/cp/cp-mv-enotsup-xattr.sh: Likewise.
* tests/cp/reflink-perm.sh: Likewise.
* tests/dd/misc.sh: Likewise.
* tests/misc/env-null.sh: Likewise.
* tests/misc/env.sh: Likewise.
* tests/misc/nice.sh: Likewise.
* tests/misc/nohup.sh: Likewise.
* tests/misc/printenv.sh: Likewise.
* tests/misc/xattr.sh: Likewise.
* tests/mv/update.sh: Likewise.
* tests/rm/deep-2.sh: Likewise.
* tests/rm/read-only.sh: Likewise.
* tests/split/r-chunk.sh: Likewise.
* tests/tail-2/follow-stdin.sh: Likewise.
* tests/tail-2/inotify-race.sh: Likewise.
* tests/tail-2/wait.sh: Likewise.
* tests/touch/no-dereference.sh: Likewise.

* cfg.mk (sc_prohibit_test_empty:): New syntax-check.
* tests/cp/proc-zero-len.sh: Adjust to avoid false syntax-check failure.
* tests/cp/proc-zero-len.sh: Likewise.
* tests/mv/part-symlink.sh: Likewise.
* tests/tail-2/infloop-1.sh: Likewise.