Lower the trial number in decaf_random

The radix_51 implementation has many more debug checks, so this takes quite
long to run, and it's rude to run a fuzzer on the CI server.

src/decaf.rs

@@ -789,7 +789,7 @@ mod test {
     #[test]
     fn decaf_random_is_valid() {
         let mut rng = OsRng::new().unwrap();
-        for _ in 0..10_000 {
+        for _ in 0..100 {
             let P = DecafPoint::random(&mut rng);
             // Check that P is on the curve
             assert!(P.0.is_valid());