refactor integration tests to run as part of build

[robzienert]

The UpdateTest and similar classes in the test source set don't actually run as part of a build. This PR refactors things around a little bit so they actually run.

Other changes:

• The plugins that this project includes in downloads were compiled against PF4J before it moved packages to org.pf4j. I couldn't find the project anywhere in GitHub that generated these, so I wrote my own and dropped the equivalents into the downloads dir: https://github.com/robzienert/pf4j-demo
• Added an .editorconfig so I could stay in sync with your conventions
• Updated to PF4J 3.2.0
• Added another REPLACE_EXISTING to installPlugin - again, this was causing subsequent runs of tests to fail. Eventually I just refactored the tests to use a temporary directory, but kept this change.
• Properly closing the Server at the completion of tests to help reduce leaking.
• Changed UpdateTest to UpdateIntegrationTest, as this class is really testing the whole project.
• Refactored PluginTest to TestPluginFixture, since this class wasn't testing anything, but appears to be used for test setup.

After doing all of this, it looks like all of the tests pass. Kind of unfortunate, because that means our integration with pf4j-update likely has a bug in it. 😆

[decebals]

It looks OK. Thanks!

[decebals]

The UpdateTest and similar classes in the test source set don't actually run as part of a build.

The UpdateTest is not a real test. Actually, it's a kind of demo about how to use pf4j-update in a pf4j based application. Probably the test directory is not the best location for this file but I didn't find a better option. After this PR, we have a broken link in readme file but I think that we can fix it relative easy.

[robzienert]

I'm not a maven guy by any means, but maven supports multi-project builds right? There could be the actual pf4j-update module and then an example module which has a compile dependency on the pf4j-update module?