random failures in the freebsd test_process_and_thread_lock unittest

[benfred]

The freebsd unittests sometimes randomly fail with

error: test failed, to rerun pass `--lib`
failures:

---- freebsd::tests::test_process_and_thread_lock stdout ----
thread 'freebsd::tests::test_process_and_thread_lock' panicked at src/freebsd/mod.rs:311:17:
assertion failed: active_thread.is_some()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    freebsd::tests::test_process_and_thread_lock

test result: FAILED. 7 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.17s

We should figure out why this test is failing and fix.

[benfred]

#97 marks this test as flaky so we can pass CI