Allow tcframe to skip checking unassigned subtask

[jonathanirvings]

Currently, tcframe always checks whether a generated testcase is also valid for subtasks not assigned to the testcase. If that's the case, tcframe will trigger an error something like

"..... satisfies subtask x but not assigned to it"

A feature suggested in this issue is allowing tcframe to skip this check by specifying some "flag" in spec.cpp.

@jonathanirvings suggested that this might be a good feature to have. With this feature, any problem with any complicated subtasks can be made simply with the following test group distribution:

TestGroup 1: Subtask {1}
TestGroup 2: Subtask {2}
....
TestGroup N: Subtask {N}

while without this feature, it is impossible to use this test group distribution if one subtask is a subset of another.

There are several testcase generator framework out there that simply takes a list of testcases to be generated for each subtask. With this feature (and using the test group distribution as illustrated above), migrating a code which uses such testcase generator framework to a code which uses tcframe becomes a lot easier.