Tests: Add RunCMake.Make case for conflicting Makefile names

Issue: #21418

Tests/RunCMake/Make/MakefileConflict.cmake

@@ -0,0 +1,5 @@
+add_custom_target(Custom)
+
+# Write a file that GNU make will prefer over "Makefile"
+# if 'cmake --build' does not explicitly specify it.
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/GNUmakefile" "")

Tests/RunCMake/Make/RunCMakeTest.cmake

@@ -42,6 +42,14 @@ run_VerboseBuild()
 run_cmake(CustomCommandDepfile-ERROR)
 run_cmake(IncludeRegexSubdir)
 
+function(run_MakefileConflict)
+  run_cmake(MakefileConflict)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/MakefileConflict-build)
+  run_cmake_command(MakefileConflict-build ${CMAKE_COMMAND} --build . --target Custom)
+endfunction()
+run_MakefileConflict()
+
 function(run_CMP0113 val)
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CMP0113-${val}-build)
   run_cmake(CMP0113-${val})