Logs help to debug flaky tests. Because sometimes just by instance logs
it is not that easy to guess what the test was doing at that time.
Especially when there is a repetitive pattern in the test.

For now instance logs and test output are printed in separate sections.
It still allows to match events by time easier compared to looking only
at instance log (it is way bigger compared to test log).

Additionally this patch sets up a `framework` package intended to host
reusable test code without polluting conftest.py

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.