Switch to test suites

[pmendelski]

Changes

Issues: #26 #126 #127

Migrates the plugin to JvmTestSuites, benefits:

• integration test source dir is automatically marked as test sources by IntelliJ
• internal objects/classes/fields from main or test sources are available in integration tests

Checklist

• [x] I have tested that there is no similar pull request already submitted
• [x] I have read contributing.md and applied to the rules
• [x] I have unit tested code changes and performed a self-review
• [x] I have tested plugin change locally on a sample project