test: add regex check in test-vm-is-context

Use a regex to validate the error message.

PR-URL: #12785
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-vm-is-context.js

@@ -26,7 +26,7 @@ const vm = require('vm');
 
 assert.throws(function() {
   vm.isContext('string is not supported');
-}, TypeError);
+}, /^TypeError: sandbox must be an object$/);
 
 assert.strictEqual(vm.isContext({}), false);
 assert.strictEqual(vm.isContext([]), false);