Stop treating 0-point cases as short-circuiting pretests

[Xyene]

After #1068, we will be able to:

• Write a script that converts existing 0-point cases into pretest_test_cases + pretest_dependencies for following test cases.
• Rejudge all affected problems so that the site can display those cases as proper pretests.
• Drop the short-circuiting behavior of 0-point testcases. This should make the codebase a little less hard to think about.