Random test failures on macOS

[matthewbauer]

We have run into a few problems on building Rustc for macOS with Nixpkgs. A few network tests seem to fail randomly. It's about 1 in every 10 tests that fail without any obvious reason.

Job: https://hydra.nixos.org/job/nixpkgs/trunk/rustc.x86_64-darwin/all

It's possible it's hardware related but has not happened on the Linux builds. Here is the full list of temperamental tests:

• net::tcp::tests::clone_while_reading
• net::tcp::tests::multiple_connect_interleaved_greedy_schedule
• net::tcp::tests::multiple_connect_interleaved_lazy_schedule
• net::tcp::tests::partial_read
• net::tcp::tests::read_eof
• net::tcp::tests::shutdown_smoke
• net::tcp::tests::tcp_clone_two_read
• net::tcp::tests::clone_accept_smoke
• run-make/issue-26092

Most of the network ones are "Address already in use":

thread '<unnamed>' panicked at 'received error for `TcpListener::bind(&addr)`: Address already in use (os error 48)', libstd/net/tcp.rs:1377:26