test: refactor test-gc-tls-external-memory

• Don’t use network connections, we don’t need them and they add more native objects into the mix when we care about other kinds of native objects.
• Run GC only once every 64 iterations – this cuts down running time from 4 s to 400 ms.
• Use common.mustCall() for the connect() handler.
• Make sure that the TLS sockets do get garbage collected, since the test would otherwise also pass if they remain alive indefinitely.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines