test: update test-run

* Added basic luacov support.
* Added use_unix_sockets_iproto option.
* Fixed TARANTOOL_SRC_DIR on >=tarantool-2.1.1-322-g3f5f59bb5.
  - It is important for app-tap/http_client.test.lua, it fails now.
* Renamed pre_cleanup to pretest_clean.
* pretest_clean: clean up _cluster space.