Merge branch 'thoennes/fix' into 'master'

with CMP0115 file endings must be explicit

See merge request !534

tests/core/CMakeLists.txt

@@ -221,7 +221,7 @@ waLBerla_compile_test( FILES VersionTest.cpp )
 waLBerla_execute_test( NAME VersionTest )
 
 if ( WALBERLA_CXX_COMPILER_IS_INTEL AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "20.0" )
-  waLBerla_compile_test( FILES VariantTest )
+  waLBerla_compile_test( FILES VariantTest.cpp )
   waLBerla_execute_test( NAME VariantTest )
 endif()