contrib: refactor preparation logic for patterns of check-code.py This is a part of preparation to apply checking with check-code.py on code fragments embedded in *.t test scripts. Before this patch, preparation logic in _preparepats() of check-code.py is not reusable. It can handle only module global list "checks". This patch splits preparation logic into small internal functions, and add the loop to invoke them, in order to increase reusability of the logic. "c[-2]" is equivalent to "c[3]" for "checks". This patch uses the former, because it will be more reusable for subsequent patch than the latter.