How do I mock a method defined in a Moo Role?

Given the following Role:

package MyRole;
use Moo::Role;

sub foo { 
    return 'blah';
}

And the following consuming class:

package MyClass;
use Moo;
with 'MyRole';

around foo { 
    my ($self, $orig) = @_;
    return 'bak' if $self->$orig eq 'baz';
    return $self->$orig;
}

I would like to test behaviour defined in the around modifier. How do I do this? It seems that Test::MockModule won't work:

use MyClass;
use Test::Most;
use Test::MockModule;

my $mock = Test::MockModule->new('MyRole');
$mock->mock('foo' => sub { return 'baz' });

my $obj = MyClass->new;
# Does not work
is $obj->foo, 'bak', 'Foo is what it oughtta be';