HydraTestContext: explicitly stop the db

This might, hopefully, I don't know, possibly force the
database to live a little while longer and *reduce* but not
eliminate errors around stopping the database before we lose all
our DB::PG handles to it.

t/lib/HydraTestContext.pm

@@ -118,6 +118,7 @@ sub DESTROY
 {
     my ($self) = @_;
     $self->db(0)->schema->storage->disconnect();
+    $self->{db_handle}->stop();
 }
 
 sub write_file {