Demolish warnings-removal tweaks

Per pull request moose#167, add comments better explaining why these changes
are necessary, and use Test::Warnings

lib/Moose/Object.pm

@@ -78,6 +78,8 @@ sub DEMOLISHALL {
 
     foreach my $class (@isa) {
         no strict 'refs';
+        # If a child module implements DEMOLISH and its parent does not
+        # then the access below can be the only reference to that parent's sub
         my $demolish = do {
             no warnings 'once';
             *{"${class}::DEMOLISH"}{CODE};

t/bugs/DEMOLISH_warnings.t

@@ -2,14 +2,14 @@ use strict;
 use warnings;
 
 use lib 't/lib';
-use Test::More tests => 1;
-
-my @warnings;
-BEGIN {
-    $SIG{__WARN__} = sub { push @warnings, @_ };
-}
+use Test::More;
+use Test::Requires qw(Test::Warnings);
+Test::Warnings->import(':no_end_test');
 
+# Demolition::OnceRemoved has a variable only in scope during the initial `use`
+# As it leaves scope, Perl will call DESTROY on it
+# (and Moose::Object will then go through its DEMOLISHALL method)
 use Demolition::OnceRemoved;
+Test::Warnings::had_no_warnings("No DEMOLISH warnings");
 
-is scalar @warnings, 0, "No warnings"
- or diag explain \@warnings;
+done_testing();

t/lib/Demolition/OnceRemoved.pm

@@ -4,6 +4,5 @@ use warnings;
 use Demolition::Demolisher;
 
 my $d = Demolition::Demolisher->new;
-$d->DEMOLISH;
 
 1;