Add frontend tests (#22783)

* basic e2e frontend test framework

* improve mock data generation and add test cases

* more cases

* add e2e tests to PR template

* don't generate mock data in PR CI

* satisfy codeql check

* fix flaky system page tab tests by guarding against crashes from incomplete mock stats

* reduce local test runs to 4 workers to match CI

web/e2e/global-setup.ts

@@ -0,0 +1,7 @@
+import { execSync } from "child_process";
+import path from "path";
+
+export default function globalSetup() {
+  const webDir = path.resolve(__dirname, "..");
+  execSync("npm run e2e:build", { cwd: webDir, stdio: "inherit" });
+}