std: Ignore a flaky test on freebsd

This test runs successfully manually, but the bots are having trouble getting
this test to pass. Ignore it on freebsd for now.
auto merge of #14046 : alexcrichton/rust/ignore-a-test-on-freebsd, r=kballard

This test runs successfully manually, but the bots are having trouble getting
this test to pass. Ignore it on freebsd for now.