Reenable lens test

It was non-deterministic. We've tried to eradicate some sources of
non-determinism recently, let's check if it fixes the test.

Author: matklad

src/test/lens.rs

@@ -15,7 +15,6 @@ use crate::{
 };
 
 #[test]
-#[ignore] // FIXME(#925) intermittent failure
 fn test_lens_run() {
     let mut env = Environment::new("lens_run");