Add publisher for test.

[XiongKezhi]

Currently, we don't have a reliable way to test the features (e.g. status checks, checks steps) that require the involvement of an implementation. Thus, adding a LogPublisher only for testing that logs the check details or anything being published is a good way to improve our test coverage and quality.

After injecting the LogPublisher into the Jenkins instance while testing, we can later check the log to make assertions, examples are like:

checks-api-plugin/src/test/java/io/jenkins/plugins/checks/steps/PublishChecksStepITest.java

Line 28 in 75b92fb

assertThat(JenkinsRule.getLog(buildSuccessfully(job)))

[mrginglymus]

For the junit plugin I created a publisher that just stores published checks in a simple list:

https://github.com/jenkinsci/junit-plugin/blob/master/src/test/java/io/jenkins/plugins/junit/checks/JUnitChecksPublisherTest.java#L35-L66

that can be inspected after a run has finished.

Might this be something useful to move into here?