CI (Buildbot): temporarily skip almost all of the tests on FreeBSD if we are on CI and the Julia version is 1.8+

[DilumAluthge]

Because the tester_freebsd64 Buildbot job is always failing, every single commit on master has an ❌, which is really noisy and unhelpful.

Until #41955 is ready, let's temporarily skip almost all of the tests if all three of the following conditions hold:

1. We are on FreeBSD.
2. We are running on CI, as evidenced by the value of the CI environment variable.
3. The Julia version is 1.8+. (FreeBSD tests are only broken on 1.8+. We expect the FreeBSD tests to be passing on 1.6 and 1.7.)

If all three of those conditions hold, we will skip almost all of the tests. That is, we will run a few test sets, just as a sanity check, But we will skip all other test sets.

This is just a short-term temporary solution. When #41955 is ready to go, we will revert this change.

[DilumAluthge]

Note that tester_freebsd64 is still going to fail on this PR, but that is because of #42944

[ararslan]

My primary concern here is the potential to regress further. If we skipped FreeBSD on master, we wouldn't have caught #42944.

[ViralBShah]

Isn't there a way to keep freebsd64 running, but have it not contribute to the overall green?

[DilumAluthge]

My primary concern here is the potential to regress further. If we skipped FreeBSD on master, we wouldn't have caught #42944.

So, we're not actually skipping all of the tests. We still run a few test sets, which should be sufficient to catch bugs like #42944.

Actually, as an example, look at the tester_freebsd64 job on this PR. If you look at the log, the job still fails due to #42944, which means that we would still have caught #42944.

[DilumAluthge]

Isn't there a way to keep freebsd64 running, but have it not contribute to the overall green?

With Buildkite, yes. For example, we are already doing that with the tester_musl64 Buildkite job. We always run the tester_musl64 job, but it is marked as an "allowed failure", which means that even if it fails, the overall Buildkite job will still be green.

With Buildbot, I don't know if that is possible.

[ararslan]

I am actually quite close to a fix, since with Jameson's help the upstream issue with libunwind has been identified and I have an upstream PR, so this PR shouldn't be necessary after all.