gh-102837: improve test coverage for math module (#102523)

- input checks for math_1(L989), math_1a(L1023), math_2(L1064,L1071), hypot(L2682), log(L2307), ldexp(L2168), ceil(L1165), floor(L1236,L1239) and dist(L2587,L2588,L2628).
- drop inaccessible "if" branch (L3518) in perm_comb_small()
- improve fsum coverage for exceptional cases (L1433,L1438,L1451,L1497), ditto fmod(L2378)
- rewrite modf to fix inaccessible case(L2229), ditto for pow(L2988)
    
(all line numbers are wrt the main branch at 5e6661bce9)