also reduce some libcore test iteration counts

Author: RalfJung

library/core/tests/num/flt2dec/random.rs

@@ -188,7 +188,7 @@ fn exact_f32_random_equivalence_test() {
 fn exact_f64_random_equivalence_test() {
     use core::num::flt2dec::strategy::dragon::format_exact as fallback;
     // Miri is too slow
-    let n = if cfg!(miri) { 3 } else { 1_000 };
+    let n = if cfg!(miri) { 2 } else { 1_000 };
 
     for k in 1..21 {
         f64_random_equivalence_test(

library/core/tests/num/flt2dec/strategy/grisu.rs

@@ -2,6 +2,7 @@ use super::super::*;
 use core::num::flt2dec::strategy::grisu::*;
 
 #[test]
+#[cfg_attr(miri, ignore)] // Miri is too slow
 fn test_cached_power() {
     assert_eq!(CACHED_POW10.first().unwrap().1, CACHED_POW10_FIRST_E);
     assert_eq!(CACHED_POW10.last().unwrap().1, CACHED_POW10_LAST_E);

library/core/tests/slice.rs

@@ -1268,14 +1268,14 @@ fn sort_unstable() {
     use rand::{rngs::StdRng, seq::SliceRandom, Rng, SeedableRng};
 
     // Miri is too slow
-    let large_range = if cfg!(miri) { 0..0 } else { 500..510 };
+    let lens = if cfg!(miri) { (2..20).chain(0..0) } else { (2..25).chain(500..510) };
     let rounds = if cfg!(miri) { 1 } else { 100 };
 
     let mut v = [0; 600];
     let mut tmp = [0; 600];
     let mut rng = StdRng::from_entropy();
 
-    for len in (2..25).chain(large_range) {
+    for len in lens {
         let v = &mut v[0..len];
         let tmp = &mut tmp[0..len];