(core) Move report-why-tests-hang helper to core

Summary:
This helps forcibly end mocha tests when they hang, and print out
something that may help debug the situation.

Also add the generated static/bundle.css file to core/.gitignore.

Also, avoid using npm-packages-offline-cache when building core, by avoiding use of .yarnrc which turns it on.

Test Plan: Tested manually

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2788

test/mocha.opts

@@ -1,2 +1,3 @@
 --require source-map-support/register
+test/report-why-tests-hang
 test/init-mocha-webdriver

test/report-why-tests-hang.js

@@ -0,0 +1,34 @@
+/**
+ * Mocha 4 no longer forces exit of a process after tests end, so if a setTimeout() or anything
+ * else is keeping node running, tests will hang after finishing.
+ *
+ * This helper module, always included via mocha.opts, ensures we print something if that happens.
+ * We use why-is-node-running module to print something informative.
+ */
+
+/* global after */
+
+const whyIsNodeRunning = require('why-is-node-running');
+
+function report() {
+  whyIsNodeRunning();
+  console.warn("*******************************************************");
+  console.warn("Something above prevented node from exiting on its own.");
+  console.warn("*******************************************************");
+  // We want to exit, but process.exit(1) doesn't work, since mocha catches it and insists on
+  // exiting with the test status result (which may be 0, and we need to indicate failure).
+  process.kill(process.pid, 'SIGTERM');
+}
+
+after(() => {
+  // --no-exit|-E flag is interpreted by mocha-webdriver library to start REPL on failure, and we
+  // do NOT want to output a big dump about that.
+  const noexit = process.argv.includes("--no-exit") || process.argv.includes('-E');
+  if (noexit) {
+    console.log("report-why-tests-hang silenced with --no-exit flag");
+  } else {
+    // If still hanging after 5s after tests finish, say something. Unref() ensures that THIS
+    // timeout doesn't itself keep node from exiting.
+    setTimeout(report, 5000).unref();
+  }
+});