test: refactor the code in test-http-keep-alive

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

Description of change

• use common.mustCall to control the functions execution automatically
• use let and const instead of var
• use assert.strictEqual instead assert.equal