test: verify tracing channel doesn't swallow unhandledRejection

Add a test to verify that TracingChannel.tracePromise doesn't swallow
unhandledRejection events in case no then/catch handler is set by user.

PR-URL: https://github.com/nodejs/node/pull/59974
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

test/parallel/test-diagnostics-channel-tracing-channel-promise-unhandled.js

@@ -0,0 +1,38 @@
+'use strict';
+
+const common = require('../common');
+const dc = require('diagnostics_channel');
+const assert = require('assert');
+
+const channel = dc.tracingChannel('test');
+
+const expectedError = new Error('test');
+const input = { foo: 'bar' };
+const thisArg = { baz: 'buz' };
+
+process.on('unhandledRejection', common.mustCall((reason) => {
+  assert.deepStrictEqual(reason, expectedError);
+}));
+
+function check(found) {
+  assert.deepStrictEqual(found, input);
+}
+
+const handlers = {
+  start: common.mustCall(check),
+  end: common.mustCall(check),
+  asyncStart: common.mustCall(check),
+  asyncEnd: common.mustCall(check),
+  error: common.mustCall((found) => {
+    check(found);
+    assert.deepStrictEqual(found.error, expectedError);
+  })
+};
+
+channel.subscribe(handlers);
+
+// Set no then/catch handler to verify unhandledRejection happens
+channel.tracePromise(function(value) {
+  assert.deepStrictEqual(this, thisArg);
+  return Promise.reject(value);
+}, input, thisArg, expectedError);