Moved all test base classes to src/test/java

- This makes sure applications can compile in strict
mode without inheriting the JUnit GWT module

- To use our test base classes for application tests
the corresponding test-jar has to be included i.e.
<dependency>
  <groupId>org.jboss.errai</groupId>
  <artifactId>errai-cdi-client</artifactId>
  <type>test-jar</type>
  <scope>test</scope>
</dependency>