Move reusable code from ContentAssistantTestCase (jst module) to common module

Content assist in CDI Seam config xml can be tested in the same way as that for EL in jsp.
For tests in the second case there is useful class ContentAssistantTestCase - but cdi module should not depend on jst module and besides ContentAssistantTestCase is bound to JSPMultiPageEditor and AutoContentAssistantProposal which are not used in cdi. Superclass in common module is needed with all the common functionality.