Use 'trap' builtin to clean up temporaries in test scripts

Some of the test scripts create temporary files, which we remove at the
end.  Except that we usually forgot to remove them on some exit paths. To
avoid this problem in future, this modifies the scripts to use the shell's
trap 0 functionality to automatically remove the temporaries on any exit.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>