[Fusion] Migrated MergeSelectionSetRewriter Tests (#8040)

Author: michaelstaib

src/HotChocolate/Fusion-vnext/test/Fusion.Utilities.Tests/Rewriters/MergeSelectionSetRewriterTests.cs

@@ -0,0 +1,55 @@
+using HotChocolate.Language;
+using HotChocolate.Types;
+using HotChocolate.Types.Mutable.Serialization;
+
+namespace HotChocolate.Fusion.Rewriters;
+
+public class MergeSelectionSetRewriterTests
+{
+    [Fact]
+    public void Merge_Two_SelectionSets()
+    {
+        // arrange
+        var sourceText = FileResource.Open("schema1.graphql");
+        var schemaDefinition = SchemaParser.Parse(sourceText);
+        var productType = (IObjectTypeDefinition)schemaDefinition.Types["Product"];
+
+        var selectionSet1 = Utf8GraphQLParser.Syntax.ParseSelectionSet(
+            """
+            {
+                id
+                name
+                reviews {
+                    id
+                }
+            }
+            """);
+
+        var selectionSet2 = Utf8GraphQLParser.Syntax.ParseSelectionSet(
+            """
+            {
+                reviews {
+                    body
+                }
+                name
+            }
+            """);
+
+        // act
+        var rewriter = new MergeSelectionSetRewriter(schemaDefinition);
+        var rewritten = rewriter.Merge([selectionSet1, selectionSet2], productType);
+
+        // assert
+        rewritten.MatchInlineSnapshot(
+            """
+            {
+                id
+                name
+                reviews {
+                    id
+                    body
+                }
+            }
+            """);
+    }
+}