try-wait test: Check exit status from wait for killed child

We are making `ExitStatus` `#[must_use]`.  So we need to explicitly
handle the exit status here.  Handle it the same way as the values
from the repeated calls to `try_wait`.

Signed-off-by: Ian Jackson <[email protected]>

Author: ijackson

src/test/ui/try-wait.rs

@@ -31,7 +31,8 @@ fn main() {
     assert!(maybe_status.is_none());
 
     me.kill().unwrap();
-    me.wait().unwrap();
+    let status = me.wait().unwrap();
+    assert!(!status.success());
 
     let status = me.try_wait().unwrap().unwrap();
     assert!(!status.success());