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

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/multiple_device_context/CMakeLists.txt

@@ -1,8 +1,8 @@
 set(MODULE_NAME multiples)
 
 set(${MODULE_NAME}_SOURCES
-    main.c
-    test_multiple_contexts.c
+    main.cpp
+    test_multiple_contexts.cpp
     test_multiple_devices.cpp
 )
 
@@ -10,9 +10,4 @@ set(${MODULE_NAME}_SOURCES
 set_source_files_properties(
         COMPILE_FLAGS -msse2)
 
-set_source_files_properties(
-    main.c
-    test_multiple_contexts.c
-        PROPERTIES LANGUAGE CXX)
-
 include(../CMakeCommon.txt)