Implement pretest dependencies

[Xyene]

Like batch dependencies, but depending on pretests. Split out from #1068:

Write judge patch for pretest_dependencies, allowing test cases/batches to depend on certain pretests passing. pretest_dependencies should default to "all pretests" unless overridden, i.e. default behavior should be that failing any pretests short circuits the rest of your run, but that all pretests will be graded before making this determination. This would allow us to continue to express "pretests act like an upper bound on your systest score".