benchmark: refactor to remove duplicate code

The dns benchmark script contained an if-then-else branch which executed almost the same code in both cases, apart from the number of args passed to the core dns lookup function.

Checklist

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

Affected core subsystem(s)