Having two different fixtures `binary_path` and
`path_to_binary_with_webui` results in weird failures in CI:

```
FAILED test/int/test_couple.py::test_exit_after_apply - conftest.TarantoolError:
    ('ER_NO_SUCH_PROC', "Procedure 'pico._inject_error' is not defined")
```

The root cause is that one `cargo build` overrides another with
different features enabled. To resolve the issue this patch
significantly refactors the test flow:

1. Web UI tests are skipped by default. One can enable it explicitly
   with `pytest --with-webui`

2. New fixture `cargo_build` behavior depends on environment:
   - In local dev environment it rebuilds the project with required features
   - In CI it skips rebuilding and runs tests against the existing build

3. The `path_to_binary_with_webui` fixture is removed.

See also:

- https://stackoverflow.com/a/55769818/1182787
- https://docs.pytest.org/en/7.4.x/how-to/writing_hook_functions.html
- https://docs.pytest.org/en/7.4.x/how-to/mark.html
- https://docs.pytest.org/en/7.4.x/example/simple.html#control-skipping-of-tests-according-to-command-line-option

Without it the previous commit is incomplete - it may raise "address
already in use" error without telling what address exactly is is use.

Specifying the particular directory with tests
significantly speeds up their collection and execution.

Before:

```console
$ time pytest -k nothing
12 deselected in 1.36s
```

After:

```
12 deselected in 0.04s
```

1. Review `Pipfile`:
  - Remove unused `filelock`;
  - Install `mypy` - static type checker for Python.
2. Add new command `pipenv run lint`.
3. Enable `mypy` in CI. Fix reported errors in `test_basics.py`.
4. Renew readme.