Improved tests of isqrt and checked_isqrt implementations

* Inputs chosen more thoroughly and systematically.
* Checks that `isqrt` and `checked_isqrt` have equivalent results for signed
  types, either equivalent numerically or equivalent as a panic and a `None`.
* Checks that `isqrt` has numerically-equivalent results for unsigned types and
  their `NonZero` counterparts.

Author: ChaiTRex

library/core/tests/num/int_macros.rs

@@ -290,38 +290,6 @@ macro_rules! int_module {
                 assert_eq!(r.saturating_pow(0), 1 as $T);
             }
 
-            #[test]
-            fn test_isqrt() {
-                assert_eq!($T::MIN.checked_isqrt(), None);
-                assert_eq!((-1 as $T).checked_isqrt(), None);
-                assert_eq!((0 as $T).isqrt(), 0 as $T);
-                assert_eq!((1 as $T).isqrt(), 1 as $T);
-                assert_eq!((2 as $T).isqrt(), 1 as $T);
-                assert_eq!((99 as $T).isqrt(), 9 as $T);
-                assert_eq!((100 as $T).isqrt(), 10 as $T);
-            }
-
-            #[cfg(not(miri))] // Miri is too slow
-            #[test]
-            fn test_lots_of_isqrt() {
-                let n_max: $T = (1024 * 1024).min($T::MAX as u128) as $T;
-                for n in 0..=n_max {
-                    let isqrt: $T = n.isqrt();
-
-                    assert!(isqrt.pow(2) <= n);
-                    let (square, overflow) = (isqrt + 1).overflowing_pow(2);
-                    assert!(overflow || square > n);
-                }
-
-                for n in ($T::MAX - 127)..=$T::MAX {
-                    let isqrt: $T = n.isqrt();
-
-                    assert!(isqrt.pow(2) <= n);
-                    let (square, overflow) = (isqrt + 1).overflowing_pow(2);
-                    assert!(overflow || square > n);
-                }
-            }
-
             #[test]
             fn test_div_floor() {
                 let a: $T = 8;