C++: Uncomment preprocessor test cases and add addition #if test case

Note that the new test case shows that line splicing is not correctly handled
in the case of `#if`.

cpp/ql/test/library-tests/preprocessor/preprocessor/pp.cpp

@@ -68,3 +68,10 @@ int templateClassContext<T> :: val = MACRO_TEMPLATECLASSCONTEXT_REFERENCED;
 
 #define INSTANTIATION
 templateClassContext<int> tcci;
+
+#define BAR
+
+#if defined(BAR) && \
+  defined(BAR)
+#warning BAR defined
+#endif