test: improve test-gc-http-client

Calling `global.gc()` in multiple places leads to unreliability. Call it
in the interval only.

PR-URL: https://github.com/nodejs/node/pull/23145


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>