chg: test: Disable statschannel RTT tests on FreeBSD

These tests rely on somewhat precise timing, as they test that answers
arrive in a particular latency bucket within the statschannel stats.
These tests are affected by various timing and network issues on our
FreeBSD CI runners and the results are very unstable. Skip these on
FreeBSD entirely.

Merge branch 'nicki/disable-statschannel-rtt-on-freebsd' into 'main'

See merge request isc-projects/bind9!11651