tests: fix coverage

lib/datasource/docker.js

@@ -38,6 +38,7 @@ async function getAuthHeaders(registry, repository) {
       `Obtaining docker registry token for ${repository} using url ${authUrl}`
     );
     const { token } = (await got(authUrl, { json: true })).body;
+    // istanbul ignore if
     if (!token) {
       logger.warn('Failed to obtain docker registry token');
       return null;