C#: Increase fieldFlowBranchLimit in test

68014fd3bf means that more accessors are properly
extracted, and consequently the calls to `get_Item` in the test have more dispatch
targets. Increasing `fieldFlowBranchLimit` makes the test pass again.

csharp/ql/test/library-tests/dataflow/collections/CollectionFlow.ql

@@ -16,6 +16,8 @@ class Conf extends DataFlow::Configuration {
       mc.getAnArgument() = sink.asExpr()
     )
   }
+
+  override int fieldFlowBranchLimit() { result = 10 }
 }
 
 from DataFlow::PathNode source, DataFlow::PathNode sink, Conf conf