test defs: work around weird ksh behaviour w.r.t. signal handling

test defs: work around weird ksh behaviour w.r.t. signal handling

* tests/defs (is_blocked_signal): Use perl to determine whether a
signal is trapped, since trying to do it portably within the shell
means opening a nasty can of worms.

For more information and background, see:
 <http://lists.gnu.org/archive/html/bug-autoconf/2011-09/msg00004.html>
 <http://mail.opensolaris.org/pipermail/ksh93-integration-discuss/2009-February/004121.html>
 <http://www.cons.org/cracauer/sigint.html>