Add external adapter plugin boundary

tests/test_verizon_adapter.py

@@ -22,6 +22,8 @@ class VerizonAdapterTests(unittest.TestCase):
             {
                 "NPM_ENDPOINTS": "192.168.1.3",
                 "VERIZON_ENDPOINTS": "192.168.1.1",
+                "ENABLED_PLUGINS": "npm,verizon",
+                "DNS_SYNC_PLUGIN_PATH": "external_plugins",
             },
             clear=True,
         ):
@@ -29,16 +31,37 @@ class VerizonAdapterTests(unittest.TestCase):
 
     def adapter(self, provider):
         config = self.config()
-        with patch(
-            "dns_sync.plugins.adapters.verizon.VerizonProvider",
-            return_value=provider,
+
+        import dns_sync.registry as registry
+
+        with patch.dict(
+            os.environ,
+            {
+                "ENABLED_PLUGINS": "npm,verizon",
+                "DNS_SYNC_PLUGIN_PATH": "external_plugins",
+            },
+            clear=False,
         ):
-            return load_adapter(
-                "verizon",
-                config,
-                config.verizon_endpoints[0],
+            module = registry._load_external_adapter_module(
+                "verizon"
             )
 
+            if module is None:
+                raise AssertionError(
+                    "external Verizon adapter was not found"
+                )
+
+            with patch.object(
+                module,
+                "VerizonProvider",
+                return_value=provider,
+            ):
+                return load_adapter(
+                    "verizon",
+                    config,
+                    config.verizon_endpoints[0],
+                )
+
     @staticmethod
     def dns_object(
         hostname="app.example.com",
@@ -85,6 +108,8 @@ class VerizonAdapterTests(unittest.TestCase):
             {
                 "NPM_ENDPOINTS": "192.168.1.3",
                 "VERIZON_ENDPOINTS": "192.168.1.1:8443",
+                "ENABLED_PLUGINS": "npm,verizon",
+                "DNS_SYNC_PLUGIN_PATH": "external_plugins",
                 "ROUTER_URL": "https://192.168.1.254",
                 "ROUTER_PASSWORD": "not-a-real-password",
             },