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)

Modules/mathmodule.c

@@ -2195,12 +2195,10 @@ math_modf_impl(PyObject *module, double x)
     double y;
     /* some platforms don't do the right thing for NaNs and
        infinities, so we take care of special cases directly. */
-    if (!Py_IS_FINITE(x)) {
-        if (Py_IS_INFINITY(x))
-            return Py_BuildValue("(dd)", copysign(0., x), x);
-        else if (Py_IS_NAN(x))
-            return Py_BuildValue("(dd)", x, x);
-    }
+    if (Py_IS_INFINITY(x))
+        return Py_BuildValue("(dd)", copysign(0., x), x);
+    else if (Py_IS_NAN(x))
+        return Py_BuildValue("(dd)", x, x);
 
     errno = 0;
     x = modf(x, &y);
@@ -2950,7 +2948,8 @@ math_pow_impl(PyObject *module, double x, double y)
             else /* y < 0. */
                 r = odd_y ? copysign(0., x) : 0.;
         }
-        else if (Py_IS_INFINITY(y)) {
+        else {
+            assert(Py_IS_INFINITY(y));
             if (fabs(x) == 1.0)
                 r = 1.;
             else if (y > 0. && fabs(x) > 1.0)
@@ -3480,9 +3479,7 @@ static const uint8_t factorial_trailing_zeros[] = {
 static PyObject *
 perm_comb_small(unsigned long long n, unsigned long long k, int iscomb)
 {
-    if (k == 0) {
-        return PyLong_FromLong(1);
-    }
+    assert(k != 0);
 
     /* For small enough n and k the result fits in the 64-bit range and can
      * be calculated without allocating intermediate PyLong objects. */