Merge #207: Move TestEnv and tests to tests/

b8a353d chore(test): move `TestEnv` and tests to `tests/` (Luis Schwab)

Pull request description:

  Closes #199

  Moves the `TestEnv` and tests from `src/lib.rs` to `tests/`.

  ## Changelog
  ```
  ## Changed
  - Move `TestEnv` to `tests/testenv/mod.rs`
  - Move tests to `tests/*`
  ```

ACKs for top commit:
  oleonardolima:
    ACK b8a353d

Tree-SHA512: 1fb09f98591f606e97554fcfe4ff841c296afd980352e3db8ba204d44e3688ec5e160c5fd4131031baf7a08adedda75f5aeb5b2a8c2060993560dfce4aef238a

Author: luisschwab