[WIP] test: fail on broken coverage

[bcoe]

@mhdawson let's make a smoke testing build that runs make coverage-run-js.

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

[nodejs-github-bot]

@bcoe build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2330/pipeline

[thefourtheye]

@bcoe Are we building a whitelist of tests which fail with the coverage instrumentation?

[bcoe]

@thefourtheye this is the intention. @mhdawson is helping with this, and we're using this branch to test.

Also, have this pull request open bcoe/c8#59, adding the ability to make our reporting fail if a threshold isn't met -- the idea is that we can ensure coverage is running, and hasn't dropped below a conservative lower bound.