clean xtend-gen during the build
custom injector provider for CompilationTestHelper

Needed in Xtext 2.7.0, see also
https://github.com/LorenzoBettini/packtpub-xtext-book-examples
xtext.version 2.7.2 in parent pom
tycho 0.21.0
explicit maven-clean plugin

To clean xtend-gen: this has been configured in pluginManagement section
in the paren pom, but we need to "call" this plugin in the projects
javadoc for expectedType