Re-enable version_works_without_rustc on windows.

This test was ignored in rust-lang#3189
without much discussion of explaining why.

AFAICT, this test works fine on Windows on both MSVC and GNU.
Empty paths do the expected behavior (preventing cargo from running
rustc). There are some special rules on Windows about discovering the
process to run (such as searching the app's launch directory), but
I do not think that is relevant here. Confirmed by trying to run
`cargo check` in this test fails to find `rustc`.

tests/testsuite/version.rs

@@ -16,7 +16,6 @@ fn simple() {
 }
 
 #[cargo_test]
-#[cfg_attr(target_os = "windows", ignore)]
 fn version_works_without_rustc() {
     let p = project().build();
     p.cargo("version").env("PATH", "").run();