tests: don't let a fail=1 env. setting induce unwarranted test failure

tests: don't let a fail=1 env. setting induce unwarranted test failure

* cfg.mk (sc_fail_is_initialized): New rule.
Fix the offenders:
* tests/cp/acl: Set fail=0
* tests/cp/backup-is-src: Likewise.
* tests/cp/file-perm-race: Likewise.
* tests/cp/reflink-auto: Likewise.
* tests/cp/same-file: Likewise.
* tests/ln/backup-1: Likewise.
* tests/misc/su-fail: Likewise.
* tests/misc/truncate-owned-by-other: Likewise.
* tests/mkdir/p-3: Likewise.
* tests/mkdir/selinux: Likewise.
* tests/mkdir/special-1: Likewise.
* tests/mv/acl: Likewise.
* tests/mv/backup-is-src: Likewise.
* tests/mv/diag: Likewise.
* tests/mv/force: Likewise.
* tests/mv/hard-link-1: Likewise.
* tests/mv/into-self-3: Likewise.
* tests/mv/sticky-to-xpart: Likewise.
* tests/touch/now-owned-by-other: Likewise.