Publish the helper functions in test/mesos.hpp

During the review of MESOS-3063 (Example of Dynamic Reservation), we found several helper functions are duplicated if de-coupled example from internal codes.

For example, there is a helper function named UNRESERVE(const Resources& resources) in src/tests/mesos.hpp. But we can not use it to compile example code with installed Mesos binaries & headers.

// Helpers for creating unreserve operations.
inline Offer::Operation UNRESERVE(const Resources& resources)
{
  Offer::Operation operation;
  operation.set_type(Offer::Operation::UNRESERVE);
  operation.mutable_unreserve()->mutable_resources()->CopyFrom(resources);
  return operation;
}

The proposal is to move such kind of helper functions into include/mesos/, so example code, test code and framework developer can reuse those helper functions. One concern is how to manage the compatibility of those helper functions.

cc mcypark