Add content_transition unit tests

- It needs to be tested yo

spec/frontend/vue_shared/components/__snapshots__/content_transition_spec.js.snap

+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`~/vue_shared/components/content_transition.vue default shows all transitions and only default is visible 1`] = `
+<div>
+  <transition-stub
+    name="test_transition_name"
+  >
+    <div
+      data-testval="default"
+    >
+      <p>
+        Default
+      </p>
+    </div>
+  </transition-stub>
+  <transition-stub
+    name="test_transition_name"
+  >
+    <div
+      data-testval="foo"
+      style="display: none;"
+    >
+      <p>
+        Foo
+      </p>
+    </div>
+  </transition-stub>
+  <transition-stub
+    name="test_transition_name"
+  >
+    <div
+      data-testval="bar"
+      style="display: none;"
+    >
+      <p>
+        Bar
+      </p>
+    </div>
+  </transition-stub>
+</div>
+`;

spec/frontend/vue_shared/components/content_transition_spec.js

+import { groupBy, mapValues } from 'lodash';
+import { shallowMount } from '@vue/test-utils';
+import ContentTransition from '~/vue_shared/components/content_transition.vue';
+
+const TEST_CURRENT_SLOT = 'default';
+const TEST_TRANSITION_NAME = 'test_transition_name';
+const TEST_SLOTS = [
+  { key: 'default', attributes: { 'data-testval': 'default' } },
+  { key: 'foo', attributes: { 'data-testval': 'foo' } },
+  { key: 'bar', attributes: { 'data-testval': 'bar' } },
+];
+
+describe('~/vue_shared/components/content_transition.vue', () => {
+  let wrapper;
+
+  afterEach(() => {
+    wrapper.destroy();
+    wrapper = null;
+  });
+
+  const createComponent = (props = {}, slots = {}) => {
+    wrapper = shallowMount(ContentTransition, {
+      propsData: {
+        transitionName: TEST_TRANSITION_NAME,
+        currentSlot: TEST_CURRENT_SLOT,
+        slots: TEST_SLOTS,
+        ...props,
+      },
+      slots: {
+        default: '<p>Default</p>',
+        foo: '<p>Foo</p>',
+        bar: '<p>Bar</p>',
+        dne: '<p>DOES NOT EXIST</p>',
+        ...slots,
+      },
+    });
+  };
+
+  const findTransitionsData = () =>
+    wrapper.findAll('transition-stub').wrappers.map((transition) => {
+      const child = transition.find('[data-testval]');
+      const { style, ...attributes } = child.attributes();
+
+      return {
+        transitionName: transition.attributes('name'),
+        isVisible: child.isVisible(),
+        attributes,
+        text: transition.text(),
+      };
+    });
+  const findVisibleData = () => {
+    const group = groupBy(findTransitionsData(), (x) => x.attributes['data-testval']);
+
+    return mapValues(group, (x) => x[0].isVisible);
+  };
+
+  describe('default', () => {
+    beforeEach(() => {
+      createComponent();
+    });
+
+    it('shows all transitions and only default is visible', () => {
+      expect(wrapper.element).toMatchSnapshot();
+    });
+
+    it('render transitions for each slot', () => {
+      expect(findTransitionsData()).toEqual([
+        {
+          attributes: {
+            'data-testval': 'default',
+          },
+          isVisible: true,
+          text: 'Default',
+          transitionName: 'test_transition_name',
+        },
+        {
+          attributes: {
+            'data-testval': 'foo',
+          },
+          isVisible: false,
+          text: 'Foo',
+          transitionName: 'test_transition_name',
+        },
+        {
+          attributes: {
+            'data-testval': 'bar',
+          },
+          isVisible: false,
+          text: 'Bar',
+          transitionName: 'test_transition_name',
+        },
+      ]);
+    });
+  });
+
+  describe('with currentSlot=foo', () => {
+    beforeEach(() => {
+      createComponent({ currentSlot: 'foo' });
+    });
+
+    it('should only show the foo slot', () => {
+      expect(findVisibleData()).toEqual({
+        default: false,
+        foo: true,
+        bar: false,
+      });
+    });
+  });
+});