benchmark: fix hanging dgram/array-vs-concat.js

Checklist

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

Affected core subsystem(s)

bencmark, dgram

Description of change

Exit process so benchmark can continue.

process.exit() was removed previously at https://github.com/nodejs/node/commit/83432bfff1e21797e497aacf4c473db1345f6334#diff-1cc47d7eb19f32771e7d3f6d813eb92cL242

This change adds it back, but only in this benchmark and not in benchmark/common.js.

/cc @mscdex