ctdb-tests: Make the debug hung script test cope with unreadable stacks

ctdb-tests: Make the debug hung script test cope with unreadable stacks

Ideally this would just involve using "test -r".  However, operating
system security features may mean that kernel stacks are not readable
even though they appear to be.

Instead, try reading that stack of a process on the test node.  If
that succeeds then so should reading the stack of the "stuck" sleep
process in the test.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13684

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Tim Beale <timbeale@catalyst.net.nz>
Autobuild-User(master): Tim Beale <timbeale@samba.org>
Autobuild-Date(master): Thu Nov 15 08:15:32 CET 2018 on sn-devel-144