Add docs for frontend integration + opt-in for Jenkins integration

[timja]

Fixes #874

Restores the ability to use Jenkins to report test failures (broken in #1008) and implements support for linting.

See also discussion in #1015 (comment) where it was decided that the restoring of the support will be opt-in to ensure that by default plugins builds will not silently fail on frontend test / lint failures.

Testing done

Tested with a local SNAPSHOT in jenkinsci/workflow-cps-plugin#935

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

Sounds OK. Is there a quick way to determine whether a given repo contains relevant tests, like a test -d something? Are there any other @jenkinsci plugins misconfigured? (There might be some @cloudbees plugins affected, so it would be nice to have a simple thing to search for in GitHub.)

[timja]

Sounds OK. Is there a quick way to determine whether a given repo contains relevant tests, like a test -d something? Are there any other @jenkinsci plugins misconfigured? (There might be some @cloudbees plugins affected, so it would be nice to have a simple thing to search for in GitHub.)

This search brings up the plugins that could be affected:
https://github.com/search?q=org%3Ajenkinsci%20mvntest&type=code

Not sure of an easy way to tell if misconfigured as you can configure it in multiple different ways.
e.g. at first glance I thought this was misconfigured:
https://github.com/jenkinsci/active-choices-plugin/blob/6736e8c22d18cadd5635410589522cd85c2be333/package.json#L12

but if you look further down you'll see its been configured a different way:
https://github.com/jenkinsci/active-choices-plugin/blob/6736e8c22d18cadd5635410589522cd85c2be333/package.json#L44-L67

[timja]

Looking through the list further I saw a few categories:

1. empty string or exit 0 defined for mvntest
2. jest with junit-jest setup
3. jjsbuilder
4. gulp
5. eslint - these are mostly misconfigured
   1. e.g. https://github.com/jenkinsci/workflow-cps-plugin/blob/0767b4ba0bfaff516d26190f151676b116cc6e61/plugin/package.json#L14
   2. https://github.com/jenkinsci/job-dsl-plugin/blob/616477fe247803b7d45f8be2d2e54b5f84ee0450/job-dsl-plugin/package.json#L8

For eslint there's an example in Jenkins core using the checkstyle format: https://github.com/jenkinsci/jenkins/blob/master/package.json#L18

I suspect anything using jjsbuilder or gulp will not be configured right and is likely unmaintained / very old.

[timja]

I'll add an eslint example before taking this out of draft and add it to workflow-cps

[jglick]

jenkinsci/workflow-cps-plugin#935 (comment)