Add dependencies and configuration for Enzyme UI tests

package.json

@@ -45,6 +45,8 @@
     "dom-seek": "^4.0.3",
     "dompurify": "^1.0.10",
     "end-of-stream": "^1.1.0",
+    "enzyme": "^3.8.0",
+    "enzyme-adapter-preact-pure": "^1.9.0",
     "escape-html": "^1.0.3",
     "escape-string-regexp": "^1.0.5",
     "eslint": "^5.12.1",

src/sidebar/test/bootstrap.js

@@ -13,3 +13,9 @@ sinon.assert.expose(assert, { prefix: null });
 window.jQuery = window.$ = require('jquery');
 require('angular');
 require('angular-mocks');
+
+// Configure Enzyme for UI tests.
+require('preact/debug');
+const { configure } = require('enzyme');
+const { Adapter } = require('enzyme-adapter-preact-pure');
+configure({ adapter: new Adapter() });