C#: Add some more tests for cs/coalesce-of-identical-expressions and update expected test output.

Author: michaelnebel

csharp/ql/test/query-tests/Language Abuse/UselessNullCoalescingExpression/UselessNullCoalescingExpression.cs

@@ -12,6 +12,8 @@ void Main(int? param)
         a = param ?? param;      // BAD
         a = a ?? use(a);         // BAD
         a = Field ?? this.Field; // BAD
+        a ??= a;                 // BAD
+        a ??= b = a;             // BAD
 
         a = a ?? cache(ref a);   // GOOD
         a = a ?? store(out a);   // GOOD
@@ -23,6 +25,7 @@ void Main(int? param)
               ?? a;              // GOOD
         a = a ?? store(out a)
               ?? a;              // GOOD
+        a ??= param;             // GOOD
     }
 
     int? cache(ref int? a)

csharp/ql/test/query-tests/Language Abuse/UselessNullCoalescingExpression/UselessNullCoalescingExpression.expected

@@ -4,3 +4,5 @@
 | UselessNullCoalescingExpression.cs:12:13:12:26 | ... ?? ... | Both operands of this null-coalescing expression access the same variable or property. |
 | UselessNullCoalescingExpression.cs:13:13:13:23 | ... ?? ... | Both operands of this null-coalescing expression access the same variable or property. |
 | UselessNullCoalescingExpression.cs:14:13:14:31 | ... ?? ... | Both operands of this null-coalescing expression access the same variable or property. |
+| UselessNullCoalescingExpression.cs:15:9:15:15 | ... ?? ... | Both operands of this null-coalescing expression access the same variable or property. |
+| UselessNullCoalescingExpression.cs:16:9:16:19 | ... ?? ... | Both operands of this null-coalescing expression access the same variable or property. |