Racy failures in test_wrappers

[cuviper]

This assertion occasionally fails:

autocfg/tests/wrappers.rs

Lines 45 to 50 in c10bc17

	// Finally, make sure that `RUSTC_WRAPPER` is applied outermost
	// by using something that doesn't pass through at all.
	env::set_var("RUSTC_WRAPPER", "/bin/true");
	env::set_var("RUSTC_WORKSPACE_WRAPPER", "/bin/false");
	let ac = autocfg::AutoCfg::new().unwrap();
	assert!(ac.probe_type("mesize")); // anything goes!

I found that the probe sometimes gets an EPIPE when writing input to the child process. In this particular test scenario, the dummy /bin/true wrapper may exit too quickly, closing its stdin before we had a chance to write anything (which it wasn't going to look at anyway).

I don't think it's wrong for the library to consider that a failed probe, but I'd like to make the test more robust.