tests: Remove priv-check. Use new require_root_ function instead.

tests: Remove priv-check.  Use new require_root_ function instead.

* tests/priv-check: Remove file.
* tests/Makefile.am (EXTRA_DIST): Remove priv-check.
* tests/test-lib.sh (require_root_): New function.
Use this function rather than sourcing the priv-check file.
* tests/sample-test: Use require_root_ rather than priv-check.
* tests/chown/basic: Likewise.
* tests/cp/cp-a-selinux: Likewise.
* tests/cp/preserve-gid: Likewise.
* tests/cp/special-bits: Likewise.
* tests/ls/nameless-uid: Likewise.
* tests/misc/chcon: Likewise.
* tests/mkdir/writable-under-readonly: Likewise.
* tests/mv/sticky-to-xpart: Likewise.
* tests/rm/fail-2eperm: Likewise.
* tests/rm/no-give-up: Likewise.
* tests/rm/one-file-system: Likewise.
* tests/tail-2/append-only: Likewise.
* tests/touch/now-owned-by-other: Likewise.
* tests/rm/fail-eperm: Use skip_if_root_ rather than priv-check.
* Makefile.maint (sc_root_tests): Reflect this change:
search for the new function name.