Make test 'iter_panic_fuse' deterministic

Fixes rayon-rs#667

'panic_fuse' will only stop other threads 'as soon as possible' -
if the other threads are fast enough, they might end up processing
the entire rest of the iterator.

This commit changes the test 'iter_panic_fuse' to properly take this
into account, by creating a custom threadpool with only 1 thread.
This makes the test deterministic - with only one thread, the panic
is guaranmteed to be observed when the next item is processed, causing
the desired early exit.

tests/iter_panic.rs

@@ -1,4 +1,6 @@
 extern crate rayon;
+#[macro_use]
+extern crate more_asserts;
 
 use rayon::prelude::*;
 use std::ops::Range;
@@ -22,26 +24,30 @@ fn iter_panic() {
 
 #[test]
 fn iter_panic_fuse() {
-    fn count(iter: impl ParallelIterator + UnwindSafe) -> usize {
-        let count = AtomicUsize::new(0);
-        let result = panic::catch_unwind(|| {
-            iter.for_each(|_| {
-                count.fetch_add(1, Ordering::Relaxed);
+    // We only use a single thread in order to make the behavior
+    // of 'panic_fuse' deterministic
+    rayon::ThreadPoolBuilder::new().num_threads(1).build().unwrap().install(|| {
+        fn count(iter: impl ParallelIterator + UnwindSafe) -> usize {
+            let count = AtomicUsize::new(0);
+            let result = panic::catch_unwind(|| {
+                iter.for_each(|_| {
+                    count.fetch_add(1, Ordering::Relaxed);
+                });
             });
-        });
-        assert!(result.is_err());
-        count.into_inner()
-    }
-
-    // Without `panic_fuse()`, we'll reach every item except the panicking one.
-    let expected = ITER.len() - 1;
-    let iter = ITER.into_par_iter().with_max_len(1);
-    assert_eq!(count(iter.clone().inspect(check)), expected);
-
-    // With `panic_fuse()` anywhere in the chain, we'll reach fewer items.
-    assert!(count(iter.clone().inspect(check).panic_fuse()) < expected);
-    assert!(count(iter.clone().panic_fuse().inspect(check)) < expected);
-
-    // Try in reverse to be sure we hit the producer case.
-    assert!(count(iter.clone().panic_fuse().inspect(check).rev()) < expected);
+            assert!(result.is_err());
+            count.into_inner()
+        }
+
+        // Without `panic_fuse()`, we'll reach every item except the panicking one.
+        let expected = ITER.len() - 1;
+        let iter = ITER.into_par_iter().with_max_len(1);
+        assert_eq!(count(iter.clone().inspect(check)), expected);
+
+        // With `panic_fuse()` anywhere in the chain, we'll reach fewer items.
+        assert_lt!(count(iter.clone().inspect(check).panic_fuse()), expected);
+        assert_lt!(count(iter.clone().panic_fuse().inspect(check)), expected);
+
+        // Try in reverse to be sure we hit the producer case.
+        assert_lt!(count(iter.clone().panic_fuse().inspect(check).rev()), expected);
+    });
 }