Add libsignal-bridge-testing for Node

Unlike Java, just bundle it into the exiting signal_node shared library. We 
don't care as much about code size here and splitting it into a separate 
library is significantly more complicated (though it might be worth it some 
day).

node/ts/test/NativeTest.ts

@@ -0,0 +1,17 @@
+//
+// Copyright 2023 Signal Messenger, LLC.
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+
+import { assert, use } from 'chai';
+import * as chaiAsPromised from 'chai-as-promised';
+import * as Native from '../../Native';
+
+use(chaiAsPromised);
+
+describe('Native', () => {
+  it('has test-only functions', () => {
+    const value = Native.test_only_fn_returns_123();
+    assert.equal(value, 123);
+  });
+});