C#: Update the viable callables test as += now is an operator call.

Author: michaelnebel

csharp/ql/test/library-tests/dispatch/viableCallable.expected

@@ -271,6 +271,7 @@
 | ViableCallable.cs:311:9:311:15 | call to method M<Int32> | M`1 | C7`1 |
 | ViableCallable.cs:314:9:314:20 | call to method M<Int32> | M`1 | C7`1 |
 | ViableCallable.cs:317:9:317:20 | call to method M<Int32> | M`1 | C6`2 |
+| ViableCallable.cs:365:9:365:15 | ... += ... | + | Int32 |
 | ViableCallable.cs:367:9:367:14 | dynamic call to method M | M | C11 |
 | ViableCallable.cs:369:9:369:18 | dynamic object creation of type C11 | C11 | C11 |
 | ViableCallable.cs:392:9:392:13 | call to method M | M | C13 |