Add test to extract method in interface

src/EditorFeatures/CSharpTest/ExtractMethod/ExtractMethodTests.cs

@@ -11051,5 +11051,37 @@ private static void NewMethod(ref object test, bool a)
 }";
             await TestExtractMethodAsync(code, expected, allowBestEffort: true);
         }
+
+        [WorkItem(30750, "https://github.com/dotnet/roslyn/issues/30750")]
+        [Fact, Trait(Traits.Feature, Traits.Features.ExtractMethod)]
+        public async Task ExtractMethodInInterface()
+        {
+            var code = @"
+interface Program
+{
+    void Foo();
+
+    void Test()
+    {
+        [|Foo();|]
+    }
+}";
+            var expected = @"
+interface Program
+{
+    void Foo();
+
+    void Test()
+    {
+        NewMethod();
+    }
+
+    void NewMethod()
+    {
+        Foo();
+    }
+}";
+            await TestExtractMethodAsync(code, expected);
+        }
     }
 }