[Clang-tidy] fix warning for Tscal used as loop counter

Replace for loop by a while loop.

Author: Akos299

src/tests/shammath/sphkernelsTests.cpp

@@ -82,13 +82,25 @@ inline void validate_kernel_3d(
     // is df = f' ?
     Tscal L2_sum = 0;
     Tscal step   = 0.01;
-    for (Tscal x = 0; x < Ker::Rkern; x += step) {
+
+    // for (Tscal x = 0; x < Ker::Rkern; x += step) {
+    //     Tscal diff = Ker::df(x) - shammath::derivative_upwind<Tscal>(x, dx, [](Tscal x) {
+    //                      return Ker::f(x);
+    //                  });
+    //     diff *= gen_norm3d;
+    //     L2_sum += diff * diff * step;
+    // }
+
+    Tscal x = 0;
+    while (x < Ker::Rkern) {
         Tscal diff = Ker::df(x) - shammath::derivative_upwind<Tscal>(x, dx, [](Tscal x) {
                          return Ker::f(x);
                      });
         diff *= gen_norm3d;
         L2_sum += diff * diff * step;
+        x += step;
     }
+
     REQUIRE_FLOAT_EQUAL(L2_sum, 0, tol);
 }