test: refactor test-net-connect-buffer

• Use arrow functions, common.mustCall()
• Remove redundant console.log()s/turn them into assertions
• Use common.expectsError()

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)

test/net