Add active panel spec

ui/spec/data_explorer/reducers/dataExplorerUISpec.js

@@ -0,0 +1,11 @@
+import reducer from 'src/data_explorer/reducers/dataExplorerUI';
+import {activatePanel} from 'src/data_explorer/actions/view';
+
+describe('Chronograf.Reducers.Panel', () => {
+  it('can set the active panel', () => {
+    const activePanel = 123;
+    const actual = reducer({}, activatePanel(activePanel));
+
+    expect(actual).to.deep.equal({activePanel});
+  });
+});