tests: GThreadPool free-function is not reliably called on FreeBSD yet
This new feature in 2.69.0 does not appear to be reliable on the FreeBSD CI runners. Disable this part of the test while it's investigated, so that we can use CI as an acceptance gate for unrelated branches again.
Leave it enabled when run with
-m thoroughso that FreeBSD developers can debug the intermittent failure.
Mitigates: #2456 (closed)
This is obviously not really desirable, but having CI that we can't reliably use as a gate, because developers mash "Retry" until their unrelated branch accidentally succeeds, doesn't seem great either.
The hack applied here is FreeBSD-specific, so that if the GThreadPool free-function regresses on other platforms like Linux or Windows, our CI gate will still detect that.