Whenever there are multiple sample test cases (which is usually the case), users have to copy paste each of them manually to the test case box to test their code before submitting. This experience should be improved. All the test cases should already be prepopulated in the test case box.

I think it'd be an excellent feature. Even if it saves 10 sec per problem per user, multiply that with the no of problems and the no of daily active users, just imagine how much time you'd end up saving for everyone.