Added monaco_loader test

spec/javascripts/repo/monaco_loader_spec.js

+describe('MonacoLoader', () => {
+  it('sets __monaco_context__', () => {
+    const monacoContext = require('monaco-editor/dev/vs/loader'); // eslint-disable-line global-require
+
+    expect(window.__monaco_context__) // eslint-disable-line no-underscore-dangle
+      .toBe(monacoContext);
+  });
+});