Refactor and rewrite testsuite.

### Changelog:
* Remove duplicate test file for `pyelliptic` (`test_bc_ec.py`).
* Refactor `pyelliptic` tests as unit tests; make them `nosetest`-ready.
* Remove execute flag from all testsuite files.
* Rewrite `test_kbucket.py` to cover almost entire of `node/kbucket.py`.
* Some minor grooming in other test files.

Please review thoroughly.