test: http2 test coverage for NghttpError

small test verifying that the NghttpError is as expected

PR-URL: https://github.com/nodejs/node/pull/15105
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

test/parallel/test-http2-util-nghttp2error.js

@@ -0,0 +1,16 @@
+// Flags: --expose-internals
+'use strict';
+
+const common = require('../common');
+const { strictEqual } = require('assert');
+const { NghttpError } = require('internal/http2/util');
+
+common.expectsError(() => {
+  const err = new NghttpError(-501);
+  strictEqual(err.errno, -501);
+  throw err;
+}, {
+  code: 'ERR_HTTP2_ERROR',
+  type: NghttpError,
+  message: 'Invalid argument'
+});