Use pytest as the testing system instead of Bats

Use pytest as the testing system instead of Bats

pytest has several benefits:

* The tests are written in Python, so I can directly import `barectf`
  and use the package to make barectf operations instead of using the
  CLI.

  Therefore I can test the package itself.

  Also, there's a single Python interpreter instance, so you don't pay
  the Python boot up cost for each test.

* With `pytest-xdist` (also a new dev dependency), you can run tests
  in parallel, for example:

      $ poetry run py.test -v -n4 tests

* It's very well known by the Python community, whereas Bats is not that
  popular.

All in all, this makes the testing process really faster (15 seconds on
my machine; used to be 1m45). It's also somewhat more enjoyable to write
Python code than Bash code to create new tests.

This patch only converts current tests using Bats to pytest.

`tests/config/yaml/conftest.py` defines a pytest fixture named
`config_fail_test`. A test receiving this fixture gets a function which
accepts a `request` fixture as well as a relative path to a YAML file
(from the test file's directory), without the `.yaml` extension. The
testing function uses the barectf API directly to try to create a
configuration from the YAML file, ensuring that
`barectf._ConfigurationParseError` is raised in the process.

`tests/config/yaml/2/test_pass_everything.py` does what
`tests/config/2/pass/everything/pass.bats` used to do, using Python's
`subprocess` module to run the C compiler and `nm` (both of which you
can override with the `CC` and `NM` environment variables).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>