[bugfix] Disable the test for long syslog messages over Unix datagram sockets on macOS

[VyrCossont]

Description

This pull request disables the test for long syslog messages over Unix datagram sockets on macOS, where it's known to hang the test runner. I suspect this is an implementation limit for macOS sockets. Given #493, which seems to cap the message size, I'm not sure this is an issue anything's going to hit in production. Also, we don't support macOS for production deployments anyway.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Thanks :)