Unit tests in core plugin

- I moved all the Unit tests classes in separate plugin fragments (needed by Tycho to execute the tests)
- Added the configuration in the root pom.xml file for executing the tests and collecting the results. There are also (commented out) settings for SonarQube.
- Fixed some version mismatches in pom and manifest files
- Removed 2 test fragment plugins that throw exception at execution time (they break the build). It is only a temporary solution.
- Removed all the test fragment plugins from the core.all feature (nothing should depend on test plugins). Do we need them in the feature?