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

Use a regex to validate the error message.

PR-URL: nodejs#12785
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Jeyanthinath

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);