Rename test .c sources to .cpp where necessary (#604)

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/api/CMakeLists.txt

@@ -1,15 +1,15 @@
 set(MODULE_NAME API)
 
 set(${MODULE_NAME}_SOURCES
-         main.c
-         test_bool.c
+         main.cpp
+         test_bool.cpp
          test_retain.cpp
-         test_retain_program.c
+         test_retain_program.cpp
          test_queries.cpp
          test_queries_compatibility.cpp
-         test_create_kernels.c
-         test_kernels.c
-         test_api_min_max.c
+         test_create_kernels.cpp
+         test_kernels.cpp
+         test_api_min_max.cpp
          test_kernel_arg_changes.cpp
          test_kernel_arg_multi_setup.cpp
          test_binary.cpp
@@ -18,9 +18,9 @@ set(${MODULE_NAME}_SOURCES
          test_create_context_from_type.cpp
          test_device_min_data_type_align_size_alignment.cpp
          test_platform.cpp
-         test_kernel_arg_info.c
-         test_kernel_arg_info_compatibility.c
-         test_null_buffer_arg.c
+         test_kernel_arg_info.cpp
+         test_kernel_arg_info_compatibility.cpp
+         test_null_buffer_arg.cpp
          test_mem_object_info.cpp
          test_queue_hint.cpp
          test_queue_properties.cpp