tests: improve integration test

[dmyger]

Suggestion: move methods start_app() and stop_app() in common utilities for integration tests.
Also, it would be useful to use Python class with destructor __exit__(self, ...) to catch and stop running instance of testing application.

See also kill_remain_processes_wrapper() method at tt/test/integration/connect/test_connect.py it kills a lot of stuck watchdog processes:
/tmp/pytest-of-runner/pytest-0/tt_build0/tt --cfg /tmp/pytest-of-runner/pytest-0/cartridge_app0/tt.yaml start --watchdog cartridge_app:***. So stopping the app should down such processes as well, too.