Removing compiler warnings.

* Summary: Removed additional 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 within test code.
* How to Verify: Using g++, build and run these tests:
    - `make test/mcmc/hmc/hamiltonians/dense_e_metric`
* Side effects: none.
* Documentation: just tests. No doc needed.
* Reviewer Suggestions: Anyone. (I've verified that the test runs on my machine. I don't think it needs the full tests -- if the reviewer agrees, we can merge.)