test_node: throw on unhandled promise rejections (#118)

This makes 'test' script treat unhandled rejections as errors.

Refs: tensorflow/tfjs-core#1152
Refs: https://nodejs.org/api/process.html#process_event_unhandledrejection

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src/run_tests.ts

@@ -28,6 +28,8 @@ import bindings = require('bindings');
 import {TFJSBinding} from './tfjs_binding';
 import {NodeJSKernelBackend} from './nodejs_kernel_backend';
 
+process.on('unhandledRejection', e => { throw e; });
+
 jasmine_util.setTestEnvs([{
   name: 'test-tensorflow',
   factory: () =>