test: refactor test/common/report.js

• Don't unnecessarily require('../common').
• Eliminate state maintained in tmppath.
• validate() was being used synchronously, but was actually asynchronous. Make it synchronous.
• Other misc. drive by cleanup.

Checklist

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