Add very basic test for global add (#1878)

* Add very basic test for global add

* Add assertion & Fix typo

__tests__/commands/global.js

@@ -0,0 +1,77 @@
+/* @flow */
+
+import {Reporter} from '../../src/reporters/index.js';
+import {run as global} from '../../src/cli/commands/global.js';
+import * as fs from '../../src/util/fs.js';
+import * as reporters from '../../src/reporters/index.js';
+import Config from '../../src/config.js';
+import assert from 'assert';
+
+jasmine.DEFAULT_TIMEOUT_INTERVAL = 90000;
+
+const stream = require('stream');
+const path = require('path');
+const os = require('os');
+
+const fixturesLoc = path.join(__dirname, '..', 'fixtures', 'global');
+
+async function runGlobal(
+  command: string,
+  flags: Object,
+  args: Array<string>,
+  name: string,
+  checkGlobal?: ?(config: Config, reporter: Reporter, out: string) => ?Promise<void>,
+): Promise<void> {
+  const dir = path.join(fixturesLoc, name);
+  const cwd = path.join(
+    os.tmpdir(),
+    `yarn-${path.basename(dir)}-${Math.random()}`,
+  );
+  await fs.unlink(cwd);
+  await fs.copy(dir, cwd);
+
+  for (const {basename, absolute} of await fs.walk(cwd)) {
+    if (basename.toLowerCase() === '.ds_store') {
+      await fs.unlink(absolute);
+    }
+  }
+
+  let out = '';
+  const stdout = new stream.Writable({
+    decodeStrings: false,
+    write(data, encoding, cb) {
+      out += data;
+      cb();
+    },
+  });
+
+  const reporter = new reporters.JSONReporter({stdout});
+
+  // create directories
+  await fs.mkdirp(path.join(cwd, '.yarn'));
+  await fs.mkdirp(path.join(cwd, 'node_modules'));
+
+  try {
+    const config = new Config(reporter);
+    await config.init({
+      cwd,
+      globalFolder: path.join(cwd, '.yarn/.global'),
+      cacheFolder: path.join(cwd, '.yarn'),
+      linkFolder: path.join(cwd, '.yarn/.link'),
+    });
+
+    await global(config, reporter, flags, [command, ...args]);
+
+    if (checkGlobal) {
+      await checkGlobal(config, reporter, out);
+    }
+
+  } catch (err) {
+    throw new Error(`${err && err.stack} \nConsole output:\n ${out}`);
+  }
+}
+test.concurrent('add without flag', (): Promise<void> => {
+  return runGlobal('add', {}, ['react-native-cli'], 'add-without-flag', async (config) => {
+    assert.ok(await fs.exists(path.join(config.cwd, 'node_modules', 'react-native-cli')));
+  });
+});