fix pinging for fake clients

[john0isaac]

Possibly fix tests

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[john0isaac]

@sh4nks it's ready for merging.

Turns out pinging clients in init is problematic. I added it as an opt-in behaviour hidden behind check_connection and disabled by default.
pallets-eco/cachelib#508

[sh4nks]

Thanks!