Allow global setup code in tests (before threads spawn)

[kamalmarhubi]

I have some setup code that I'd like to run that requires the process be single threaded when it runs. std::sync::Once called at the top of each #[test] isn't enough, as threads have already spawned by that point. My workaround is to use harness = false but I either lose nice reporting or opt into #[feature(test)] and require nightly.

I'm aware of #816, but I'm hoping that a limited improvement might be allowed to the stable interface of the existing test crate... :-)

[kamalmarhubi]

For a concrete example where this could be useful: nix-rust/nix#248

The test calls unshare(2) to enter a user namespace; this cannot be done if the calling process is threaded.