refactor away test/inspector

Refs: #13622 (closed), cc @trott @refack

Tried to handle as much as I could in this PR, but there here are some things I'd like to handle in separate PRs:

• Convert inspector-helper.js to a proper "common" module (write basic docs, etc.)
• Make sure all mentions of test/inspector are gone
• Evaluate if anymore of tests can/should be moved to test/parallel

Checklist

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

Affected core subsystem(s)

test