This better satisfies Jim Meyering’s point in:
https://lists.gnu.org/archive/html/bug-gnulib/2016-03/msg00074.html
that it’s more useful when each temporary test directory name
includes the corresponding test’s name.
* build-aux/mktempd: Try mktemp -t only with GNU syntax, so that
NetBSD mktemp fails. Also, reject templates beginning with "-" so
that they are not treated as options.
* tests/init.sh (mktempd_): Likewise.