CppEditor: Clean up followsymbol_switchmethoddecldef_test.cpp

with respect to test_FollowSymbolUnderCursor_virtualFunctionCall*
functions:

 - Use same function names in test code
 - Shorten test code
 - Mimic GenericProposalWidget::showProposal() calls more completely
   VirtualFunctionTestAssistProvider::itemList()

Change-Id: Ie1bae5f00d550a9a61981945de41daf6bfeee5ff
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>