fix(repositories): make JenkinsRepositoryAdapter more resilient

[skaldarnar]

Testing JenkinsRepositoryAdapter - fetchReleases

input	nothing (the adapter itself is initialized with a release specification)
action	IO and processing of server resource(s) to internal data
output	the list of available releases

Collaborators

• JenkinsClient - for requesting build information in internal format and fetch files
• module.* - the internal data model
• repositories.Jenkins - typed representation of the JSON model

Behavior

• fetch Jenkins build information via JenkinsClient
• process the content into a list of results
• for each element in the content
  • compute another URL from the element
  • open another connection to that URL
  • use JenkinsClient to fetch the content of the file
  • continue processing

Ideas

• [x] add abstraction for IO operations (injected collaborator) -> JenkinsClient
• [x] only test fetchReleases as a unit

[skaldarnar]

Example payload, positive case:

{
  "_class": "org.jenkinsci.plugins.workflow.job.WorkflowJob",
  "builds": [
    {
      "_class": "org.jenkinsci.plugins.workflow.job.WorkflowRun",
      "artifacts": [
        { "relativePath": "distros/omega/build/distributions/md5sums.txt" },
        { "relativePath": "distros/omega/build/distributions/sha256sums.txt" },
        {
          "relativePath": "distros/omega/build/distributions/TerasologyOmega.zip"
        }
      ],
      "number": 1,
      "result": "SUCCESS",
      "timestamp": 1604285977306,
      "url": "http://jenkins.terasology.io/teraorg/job/Nanoware/job/Omega/job/develop/1/"
    }
  ]
}

[skaldarnar]

This has become quite a behemoth of a PR for the small fixes that were actually necessary :see_no_evil: I think I learned a lot about testing (and making code testable), and I discovered more bugs while writing more tests.