Plugins: Ensure service registration occurs in right order (#74001)

* make sure service registration occurs in right order

* fix test

pkg/plugins/manager/fakes/fakes.go

@@ -583,3 +583,26 @@ func (p *FakeBackendPlugin) Kill() {
 	defer p.mutex.Unlock()
 	p.Running = false
 }
+
+type FakeFeatureToggles struct {
+	features map[string]bool
+}
+
+func NewFakeFeatureToggles(features ...string) *FakeFeatureToggles {
+	m := make(map[string]bool)
+	for _, f := range features {
+		m[f] = true
+	}
+
+	return &FakeFeatureToggles{
+		features: m,
+	}
+}
+
+func (f *FakeFeatureToggles) GetEnabled(_ context.Context) map[string]bool {
+	return f.features
+}
+
+func (f *FakeFeatureToggles) IsEnabled(feature string) bool {
+	return f.features[feature]
+}

pkg/plugins/manager/testdata/external-registration/plugin.json

@@ -0,0 +1,41 @@
+{
+  "id": "grafana-test-datasource",
+  "type": "datasource",
+  "name": "Test",
+  "backend": true,
+  "executable": "gpx_test_datasource",
+  "info": {
+    "author": {
+      "name": "Grafana Labs",
+      "url": "https://grafana.com"
+    },
+    "logos": {
+      "large": "img/ds.svg",
+      "small": "img/ds.svg"
+    },
+    "screenshots": [],
+    "updated": "2023-08-03",
+    "version": "1.0.0"
+  },
+  "externalServiceRegistration": {
+    "impersonation": {
+      "enabled" : true,
+      "groups" : true,
+      "permissions" :  [
+        {
+          "action": "read",
+          "scope": "datasource"
+        }
+      ]
+    },
+    "self": {
+      "enabled" : true,
+      "permissions" :  [
+        {
+          "action": "read",
+          "scope": "datasource"
+        }
+      ]
+    }
+  }
+}

pkg/plugins/pfs/pfs_test.go

@@ -131,6 +131,9 @@ func TestParsePluginTestdata(t *testing.T) {
 			rootid:  "grafana-worldmap-panel",
 			subpath: "plugin",
 		},
+		"external-registration": {
+			rootid: "grafana-test-datasource",
+		},
 	}
 
 	staticRootPath, err := filepath.Abs("../manager/testdata")