Moved service decorator to service helpers

Moved the service decorator to the service helpers module and moved the
associated tests.

tests/helpers/test_service.py

@@ -7,7 +7,6 @@ Test service helpers.
 import unittest
 from unittest.mock import patch
 
-from homeassistant.const import SERVICE_TURN_ON
 from homeassistant.helpers import service
 
 from tests.common import get_test_home_assistant, mock_service
@@ -23,10 +22,23 @@ class TestServiceHelpers(unittest.TestCase):
         self.hass = get_test_home_assistant()
         self.calls = mock_service(self.hass, 'test_domain', 'test_service')
 
+        service.HASS = self.hass
+
     def tearDown(self):  # pylint: disable=invalid-name
         """ Stop down stuff we started. """
         self.hass.stop()
 
+    def test_service(self):
+        """ Test service registration decorator. """
+        runs = []
+
+        decor = service.service('test', 'test')
+        decor(lambda x, y: runs.append(1))
+
+        self.hass.services.call('test', 'test')
+        self.hass.pool.block_till_done()
+        self.assertEqual(1, len(runs))
+
     def test_split_entity_string(self):
         service.call_from_config(self.hass, {
             'service': 'test_domain.test_service',