Rewrite CMakeLists for istl-solver-playground

Summary

The istl-solver-playground executable was defined in cmake in a very complicated way. This MR simplifies this. It is also a preparation for later changes in cmake where dune_add_test will work out-of-the-box, but manual executables would need adaptions

src/CMakeLists.txt

 # SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
 # SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 
-add_executable(istl-solver-playground EXCLUDE_FROM_ALL istl-solver-playground.cc)
-add_dune_all_flags(istl-solver-playground)
+dune_add_test(NAME istl-solver-playground
+  SOURCES istl-solver-playground.cc)
 dune_symlink_to_source_files(FILES playground.ini)
-dune_add_test(TARGET istl-solver-playground)