Have Lockable#advisory_locked? directly query pg_locks table

Addresses a flaky test that simply counted `pg_locks` table rows.

lib/good_job/lockable.rb

@@ -205,7 +205,15 @@ def with_advisory_lock
     # Tests whether this record has an advisory lock on it.
     # @return [Boolean]
     def advisory_locked?
-      self.class.unscoped.advisory_locked.exists?(id: send(self.class.primary_key))
+      query = <<~SQL.squish
+        SELECT 1 AS one
+        FROM pg_locks
+        WHERE pg_locks.locktype = 'advisory'
+          AND pg_locks.objsubid = 1
+          AND pg_locks.classid = ('x' || substr(md5(:table_name || :id::text), 1, 16))::bit(32)::int
+          AND pg_locks.objid = (('x' || substr(md5(:table_name || :id::text), 1, 16))::bit(64) << 32)::bit(32)::int
+      SQL
+      self.class.connection.execute(sanitize_sql_for_conditions([query, { table_name: self.class.table_name, id: send(self.class.primary_key) }])).ntuples.positive?
     end
 
     # Tests whether this record is locked by the current database session.

spec/lib/good_job/lockable_spec.rb

@@ -109,7 +109,26 @@
 
       expect do
         job.advisory_unlock
-      end.to change(PgLock, :count).by(-1)
+      end.to change(job, :advisory_locked?).from(true).to(false)
+    end
+  end
+
+  describe '#advisory_locked?' do
+    it 'reflects whether the record is locked' do
+      expect(job.advisory_locked?).to eq false
+      job.advisory_lock
+      expect(job.advisory_locked?).to eq true
+      job.advisory_unlock
+      expect(job.advisory_locked?).to eq false
+    end
+
+    it 'is accurate even if the job has been destroyed' do
+      job.advisory_lock
+      expect(job.advisory_locked?).to eq true
+      job.destroy!
+      expect(job.advisory_locked?).to eq true
+      job.advisory_unlock
+      expect(job.advisory_locked?).to eq false
     end
   end