Should we unify enumerated tests across platforms?

An idea I've toyed with is
```rust
#[test]
#[cfg(windows)]
fn test() {
}
```
being turned into
```rust
#[test]
#[cfg(windows)]
fn test() {
}

#[test]
#[cfg(not(windows))]
fn test(ctx: TestContext) {
    ctx.sklip()?;
}
```
(#48 makes some reference to this)

This would let you see what coverage you are missing when you run tests on one platform.