CppEditor: Tests: Refactor out BaseQuickFixTestCase

We will add another type of test case that allows us to test the list of
offered operations.

Change-Id: I528b3844181c139c8955091ac0bbcf6475fd8a4f
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
CppEditor: Avoid duplicate "Add #include XYZ"

Task-number: QTCREATORBUG-13422
Change-Id: I3648bf44760fdac4e8e1e79652519136af6032c8
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>