Support externally-generated test cases

[fushar]

Often, when we prepare a contest, some author proposes a problem with its corresponding test cases, generated by their own way. Usually, there are different scenarios:

1. Trust the problem author that they have validated that the test cases are valid.
2. Convert their generator into tcframe format.
3. Write an external validator and validate the test cases (without tcframe at all).

This issue proposes a fourth way: support external test case in test case specification, so that we can still declare the input format for validation, etc. Example syntax:

void TestCases() {
    EXTERNAL_CASE("external/tree_1.in", "external/tree_1.ans");
}