test: refactor test-timers-this

• use common.mustCall() and eliminate exit handler
• provide timer durtion of 1ms where previously omitted
• var -> const

Checklist

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

Affected core subsystem(s)

Description of change