Fix incorrect usage of constexpr and redundant const casts

Replaced `constexpr` with `const` for `kCount` in the performance test to align with proper usage. Removed the unnecessary `const` cast in the functional test for cleaner and more accurate type handling.

Author: allnes

tasks/example/tests/functional/main.cpp

@@ -44,7 +44,7 @@ class NesterovARunFuncTests : public ppc::util::BaseRunFuncTests<InType, OutType
     }
 
     TestType params = std::get<ppc::util::GTestParamIndex::kTestParams>(GetParam());
-    const auto k_count = static_cast<const std::vector<int>::size_type>(((width + height) / std::get<0>(params)) *
+    const auto k_count = static_cast<std::vector<int>::size_type>(((width + height) / std::get<0>(params)) *
                                                                         std::stoi(std::get<1>(params)));
     input_data_ = InType(k_count * k_count, 0);
     for (std::vector<int>::size_type i = 0; i < k_count; i++) {

tasks/example/tests/performance/main.cpp

@@ -19,7 +19,7 @@
 namespace nesterov_a_test_task {
 
 class ExampleRunPerfTest : public ppc::util::BaseRunPerfTests<InType, OutType> {
-  static constexpr std::vector<int>::size_type kCount = 400;
+  const std::vector<int>::size_type kCount = 400;
   InType input_data_;
 
   void SetUp() override {