Set up and run tests on plugin installation manager.
Move the plugins used for test to /test/resources
A fair amount of tests were created and merged in https://github.com/jenkinsci/plugin-installation-manager-tool/pull/3.
A pull request for additional fixes for testing was created: https://github.com/jenkinsci/plugin-installation-manager-tool/pull/8
TODO: After looking at the Jenkins docker plugin-installation.sh documentation, I don't think I've tested installation plugins from the incremental repository thoroughly enough.