Add Jest vue_mock_directive

**Why?**
- This is helpful at asserting that a specific value
  set on a specific directive of a component.

spec/frontend/helpers/vue_mock_directive.js

+export const getKey = name => `$_gl_jest_${name}`;
+
+export const getBinding = (el, name) => el[getKey(name)];
+
+export const createMockDirective = () => ({
+  bind(el, { name, value, arg, modifiers }) {
+    el[getKey(name)] = {
+      value,
+      arg,
+      modifiers,
+    };
+  },
+
+  unbind(el, { name }) {
+    delete el[getKey(name)];
+  },
+});