Fixes <https://bugs.gnu.org/25476>.
Reported by Paul Garlick <pgarlick@tourbillion-technology.com>.
* tests/syscalls.scm ("pivot-root"): Skip only when
PERFORM-CONTAINER-TESTS? is true. Rewrite to use a socket pair instead
of a pipe. Synchronize parent and child so that the parent can
initialize the child's UID and GID mappings before continuing.