dns: fix issues in dns benchmark

The benchmark script for dns contained functions with args declared but never used. This fix removes those arguments from the function signatures.

No test existed for the dns benchmark so one was added to the parallel suite.

The dns benchmark uses the core dns.lookup() function which does not access the network but instead uses "an operating system facility that can associate names with addresses, and vice versa" e.g. similar to ping; however, it is a synchronous call which runs on the libuv threadpool - the number of test calls was therefore reduced to 5e4 (from 5e6).

Checklist

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

Affected core subsystem(s)