tests/numeric: drop use std::f64 (1.99 fully deprecates legacy float consts)

With the module import, `f64::NAN` resolved to the deprecated
`std::f64::NAN`; without it, it is the associated constant. Four sites.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YPZhKpSxN7eSxk7VH2vzB

Author: claude

tests/numeric.rs

@@ -4,7 +4,6 @@
 
 use approx::assert_abs_diff_eq;
 use ndarray::{arr0, arr1, arr2, array, aview1, Array, Array1, Array2, Array3, Axis};
-use std::f64;
 
 #[test]
 fn test_mean_with_nan_values() {