message: fixes creation of device executors
diff: #include "core/test/utils/assertions.hpp"
  +#include "test/utils/executor.hpp"
  namespace reference {
  @@ -56,11 +57,12 @@ public:
  int& value;
  };
  +class Executor : public CommonTestFixture {};
  -TEST(Executor, RunsCorrectOperation)
  +
  +TEST_F(Executor, RunsCorrectOperation)
  {
  int value = 0;
  - auto exec = gko::EXEC_TYPE::create();
  exec->run(ExampleOperation(value));
  @@ -71,14 +73,13 @@ TEST(Executor, RunsCorrectOperation)
  #ifndef GKO_COMPILING_REFERENCE
  -TEST(Executor, RunsCorrectLambdaOperation)
  +TEST_F(Executor, RunsCorrectLambdaOperation)
  {
  int value = 0;
  auto omp_lambda = [&value]() { value = omp::value; };
  auto cuda_lambda = [&value]() { value = cuda::value; };
  auto hip_lambda = [&value]() { value = hip::value; };
  auto dpcpp_lambda = [&value]() { value = dpcpp::value; };
  - auto exec = gko::EXEC_TYPE::create();
  exec->run(omp_lambda, cuda_lambda, hip_lambda, dpcpp_lambda);
  