C++: Add a test case resembling the example from ODASA-3940.

cpp/ql/test/query-tests/Critical/SizeCheck/SizeCheck.expected

@@ -2,3 +2,4 @@
 | test.c:17:20:17:25 | call to malloc | Type 'double' is 8 bytes, but only 5 bytes are allocated. |
 | test.c:32:19:32:24 | call to malloc | Type 'float' is 4 bytes, but only 2 bytes are allocated. |
 | test.c:33:20:33:25 | call to malloc | Type 'double' is 8 bytes, but only 4 bytes are allocated. |
+| test.c:59:15:59:20 | call to malloc | Type 'MyUnion' is 128 bytes, but only 8 bytes are allocated. |

cpp/ql/test/query-tests/Critical/SizeCheck/SizeCheck2.expected

@@ -6,3 +6,4 @@
 | test.c:17:20:17:25 | call to malloc | Allocated memory (5 bytes) is not a multiple of the size of 'double' (8 bytes). |
 | test.c:32:19:32:24 | call to malloc | Allocated memory (2 bytes) is not a multiple of the size of 'float' (4 bytes). |
 | test.c:33:20:33:25 | call to malloc | Allocated memory (4 bytes) is not a multiple of the size of 'double' (8 bytes). |
+| test.c:59:15:59:20 | call to malloc | Allocated memory (8 bytes) is not a multiple of the size of 'MyUnion' (128 bytes). |

cpp/ql/test/query-tests/Critical/SizeCheck/test.c

@@ -43,5 +43,18 @@ void good1(void) {
     free(dptr);
 }
 
+typedef struct _myStruct
+{
+	int x, y;
+} MyStruct;
 
+typedef union _myUnion
+{
+	MyStruct ms;
+	char data[128];
+} MyUnion;
 
+void test_union() {
+	MyUnion *a = malloc(sizeof(MyUnion)); // GOOD
+	MyUnion *b = malloc(sizeof(MyStruct)); // BAD (too small)
+}