Re-enable killing_cargo_releases_the_lock on windows.

AFAICT, we do not test on these older platforms anymore.
Regardless, the test seems to work fine on 32-bit windows-gnu
on Windows 10.

See rust-lang#3102 (comment)
where it was originally disabled.

tests/testsuite/concurrent.rs

@@ -351,9 +351,7 @@ fn same_project() {
 
 // Make sure that if Cargo dies while holding a lock that it's released and the
 // next Cargo to come in will take over cleanly.
-// older win versions don't support job objects, so skip test there
 #[cargo_test]
-#[cfg_attr(target_os = "windows", ignore)]
 fn killing_cargo_releases_the_lock() {
     let p = project()
         .file(