test: fix flaky test-zlib-unused-weak.js

PR-URL: #38149
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

test/parallel/test-zlib-unused-weak.js

@@ -6,6 +6,7 @@ const zlib = require('zlib');
 
 // Tests that native zlib handles start out their life as weak handles.
 
+global.gc();
 const before = process.memoryUsage().external;
 for (let i = 0; i < 100; ++i)
   zlib.createGzip();