Speed up test suite by using local npm.js stub

modules/__mocks__/npmMock.js

@@ -0,0 +1,33 @@
+import fs from 'fs';
+import path from 'path';
+
+function getPackageInfo(packageName) {
+  const file = path.resolve(__dirname, `./metadata/${packageName}.json`);
+
+  try {
+    return JSON.parse(fs.readFileSync(file, 'utf-8'));
+  } catch (error) {
+    return null;
+  }
+}
+
+export function getVersionsAndTags(packageName) {
+  const info = getPackageInfo(packageName);
+  return info
+    ? { versions: Object.keys(info.versions), tags: info['dist-tags'] }
+    : [];
+}
+
+export function getPackageConfig(packageName, version) {
+  const info = getPackageInfo(packageName);
+  return info ? info.versions[version] : null;
+}
+
+export function getPackage(packageName, version) {
+  const file = path.resolve(
+    __dirname,
+    `./packages/${packageName}-${version}.tgz`
+  );
+
+  return fs.existsSync(file) ? fs.createReadStream(file) : null;
+}