C++: Add test cases for constant initializers

Adds test cases for initialisation of constants which aren't simple
zeros.  Example: int x = int();

cpp/ql/test/duplication-tests/constants/constants.cpp

@@ -0,0 +1,12 @@
+
+int x = int();
+float y = float();
+double z = double();
+
+/* This produces a getValueText() of 0 for R() in line 9, which is debatable.  */
+struct R {};
+struct S {
+  S() : S(R()) { }
+  S(R) { }
+};
+S s;

cpp/ql/test/duplication-tests/constants/expr.expected

@@ -0,0 +1,4 @@
+| constants.cpp:2:9:2:13 | 0 | int() |
+| constants.cpp:3:11:3:17 | 0.0 | float() |
+| constants.cpp:4:12:4:19 | 0.0 | double() |
+| constants.cpp:9:11:9:13 | 0 | 0 |

cpp/ql/test/duplication-tests/constants/expr.ql

@@ -0,0 +1,4 @@
+import cpp
+
+from Expr e
+select e, e.getValueText()