tests: enable nock when needed

by default, it makes all requests disappear

src/test/updatechecker-test.js

@@ -21,6 +21,7 @@ describe('updatechecker', function () {
     const { setup, cleanup, app, appstoreToken, mockApiServerOrigin } = common;
 
     before(setup);
+    before(() => { if (!nock.isActive()) nock.activate(); });
     after(cleanup);
 
     describe('box', function () {