Allow tests to be ignored conditionally

[brson]

Right now we can ignore a test like

#[test]
#[ignore]
fn test() { /* do something */ }

But to ignore it only on windows you've got to do

#[test]
#[cfg(target_os = "win32")]
#[ignore]
fn test() { /* do nothing */ }

#[test]
#[cfg(target_os = "macos")]
#[cfg(target_os = "linux")]
fn test() { /* do something */ }

We should just be able to do this

#[test]
#[ignore(target_os = "win32")]
fn test() { /* do something */ }

Where the thing insider ignore() is just matched against all configuration items in the compilation environment