test: deflake test-vm-contextified-script-leak

Similar to the test-vm-source-text-module-leak fix, use a snapshot
to force a thorough GC in order to prevent false positives.

PR-URL: nodejs#49710
Refs: nodejs/reliability#669
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: joyeecheung

test/es-module/test-vm-contextified-script-leak.js

@@ -13,4 +13,4 @@ async function createContextifyScript() {
     async importModuleDynamically() {},
   });
 }
-checkIfCollectable(createContextifyScript, 2048);
+checkIfCollectable(createContextifyScript, 2048, 512);