test: refactor test-net-server-max-connections

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test net

Description of change

The test timed out on Windows in CI. Made the following changes:

• reduced total connections from 200 to 20
• var -> const
• string concatenation -> templates
• assert.equal -> assert.strictEqual

/cc @nodejs/testing