Merge branch 'issue/istl-playground-as-test' into 'master'

Rewrite CMakeLists for istl-solver-playground

See merge request core/dune-istl!507

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)