ci: use `coverageReport` in Jenkinsfile

[trentm]

Assuming we want it, we should be able to get npm run coverage to generate a coverage report in a format used by coverageReport. As of this apm-pipeline-library change, this would result in a coverage report GitHub comment (like this one) on PRs. This could be useful as a hint/prod/success-metric.

The format needed is, I think, the cobertura-compatible XML format, which our coverage tool (nyc, which uses istanbul) supports: https://istanbul.js.org/docs/advanced/alternative-reporters/#cobertura

Perhaps use this part of the apm-agent-python/Jenkinsfile as a guide for how to use coverageReport.