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

Failed test: https://ci.nodejs.org/job/node-test-commit-linux/nodes=alpine-last-latest-x64/40825/testReport/junit/(root)/test/parallel_test_zlib_unused_weak/

AssertionError [ERR_ASSERTION]: Expected after-GC delta -29156 to be less than 5 % of before-GC delta -9751

I think the delta should be positive. Calling gc() before zlib.createGzip() should make it more stable.