Remove compiler warnings.

* Summary: Removed compiler warnings in tests. This only touches new tests written in #341.
* Intended Effect: No change to code / test behavior. Only removes compiler warnings. This was accomplished by changing int to size_t in loops.
* How to Verify: Using g++, build and run these tests:
    - `make test/mcmc/hmc/hamiltonians/diag_e_metric`
    - `make test/mcmc/hmc/hamiltonians/unit_e_metric`
    - `make test/mcmc/hmc/integrators/expl_leapfrog`
* Side effects: none.
* Documentation: just tests. No doc needed.
* Reviewer Suggestions: @betanalpha. See the changes made and be aware of it for future pull requests.