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

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/buffers/CMakeLists.txt

@@ -1,17 +1,17 @@
 set(MODULE_NAME BUFFERS)
 
 set(${MODULE_NAME}_SOURCES
-    main.c
-    test_buffer_copy.c
-    test_buffer_read.c
-    test_buffer_write.c
-    test_buffer_mem.c
-    array_info.c
-    test_buffer_map.c
+    main.cpp
+    test_buffer_copy.cpp
+    test_buffer_read.cpp
+    test_buffer_write.cpp
+    test_buffer_mem.cpp
+    array_info.cpp
+    test_buffer_map.cpp
     test_sub_buffers.cpp
-    test_buffer_fill.c
-    test_buffer_migrate.c
-    test_image_migrate.c
+    test_buffer_fill.cpp
+    test_buffer_migrate.cpp
+    test_image_migrate.cpp
 )
 
 include(../CMakeCommon.txt)