Integrate `trustme` in tests

[webknjaz]

Currently, there's https://github.com/ansible-community/antsibull/blob/21eed1c4a2885ad168f40f7e40ad4904d2f3d7c6/tests/certificate_utils.py vendored with TLS helpers for tests.

There's a better solution with a nice API: https://trustme.readthedocs.io/.

Example pytest fixture integrations: https://github.com/aio-libs/aiohttp/blob/2cad224c49e2791ceb5873b8825fe5b72d787272/tests/conftest.py#L34-L84 + https://github.com/aio-libs/aiohttp/blob/2cad224c49e2791ceb5873b8825fe5b72d787272/tests/test_client_functional.py#L267-L284.