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

Remove hacks to force language from CMake files.

Closes KhronosGroup/OpenCL-CTS#25

test_conformance/integer_ops/CMakeLists.txt

@@ -1,16 +1,16 @@
 set(MODULE_NAME INTEGER_OPS)
 
 set(${MODULE_NAME}_SOURCES
-    main.c
-    test_int_basic_ops.c
+    main.cpp
+    test_int_basic_ops.cpp
     test_integers.cpp
     test_upsample.cpp
-    test_intmul24.c test_intmad24.c
-    test_sub_sat.c test_add_sat.c
-    test_abs.c test_absdiff.c
+    test_intmul24.cpp test_intmad24.cpp
+    test_sub_sat.cpp test_add_sat.cpp
+    test_abs.cpp test_absdiff.cpp
     test_unary_ops.cpp
-    verification_and_generation_functions.c
-    test_popcount.c
+    verification_and_generation_functions.cpp
+    test_popcount.cpp
 )
 
 include(../CMakeCommon.txt)