test: 💍 add another test for valid suffix with weird exponent

Author: richard-uk1

tests/ui/consts/custom-suffixes.rs

@@ -19,4 +19,5 @@ fn main() {
     const _M: f64 = 1.0e+________________________6;
     const _N: f64 = 1.0e-________________________6;
     const _O: f64 = 1.0e-________________________9;
+    const _P: f64 = 1e_______________0f64;
 }