[cmake] fix adding tests to format

cmake/modules/DuneXtCommonMacros.cmake

@@ -50,7 +50,9 @@ macro(add_format)
     find_program(FORMAT NAMES clang-format clang-format-3.4 clang-format-3.5 clang-format-3.6 clang-format-3.7 clang-format-3.8 clang-format-3.9)
     if(EXISTS ${FORMAT})
         message(STATUS "adding format target")
-        add_custom_target( format SOURCES ${ARGN} )
+	if (NOT TARGET format)
+            add_custom_target( format SOURCES ${ARGN} )
+        endif (NOT TARGET format)
         foreach(_file ${ARGN})
             string(REPLACE "/" "_" fn ${_file})
             if(NOT EXISTS ${_file})

dune/xt/CMakeLists.txt

@@ -27,7 +27,7 @@ dune_library_add_sources(dunextcommon SOURCES ${lib_dune_xt_common_sources})
 add_analyze(${lib_dune_xt_common_sources})
 FILE( GLOB_RECURSE _header "${CMAKE_CURRENT_SOURCE_DIR}/*.hh" )
 
-add_format(${lib_dune_xt_common_sources} ${test_sources} ${_header})
+add_format(${lib_dune_xt_common_sources} ${_header})
 install(DIRECTORY ${DUNE_XT_COMMON_TEST_DIR}/gtest
         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/xt/test/
         FILES_MATCHING PATTERN "*")

dune/xt/common/test/CMakeLists.txt

@@ -14,5 +14,7 @@ DEPENDENCYCHECK( ${xtcommon} )
 set_source_files_properties( ${DUNE_HEADERS} PROPERTIES HEADER_FILE_ONLY 1 )
 
 BEGIN_TESTCASES(dunextcommon)
- 
-END_TESTCASES()
\ No newline at end of file
+
+add_format(${test_sources})
+
+END_TESTCASES()