Tests are installed

Hi! I package this project for Arch Linux.

I noticed, that all test files are installed when installing this package. Although the test files should be part of an sdist or source tarball, they should not be installed to the target system, as tests should be run beforehand.

Moving the various `*_test.py` and `testutils.py` files from the `fire/` directory to a dedicated `tests/` directory would help you to ignore them for installation.