Add SA1101 tests for extension foreach

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/ReadabilityRules/SA1101CSharp9UnitTests.cs

@@ -33,5 +33,76 @@ public A UpdateA(A value)
 
             await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
+
+        [Fact]
+        [WorkItem(3976, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3976")]
+        public async Task TestForeachWithExtensionEnumeratorAsync()
+        {
+            var testCode = @"
+using System.Collections.Generic;
+
+public class TestClass
+{
+    private readonly ExtensionEnumerable values = new();
+
+    public void Test()
+    {
+        foreach (var value in [|values|])
+        {
+        }
+    }
+}
+
+public class ExtensionEnumerable
+{
+}
+
+public struct ExtensionEnumerator
+{
+    public int Current => 0;
+
+    public bool MoveNext() => false;
+}
+
+public static class ExtensionEnumerableExtensions
+{
+    public static ExtensionEnumerator GetEnumerator(this ExtensionEnumerable enumerable) => new();
+}
+";
+
+            var fixedCode = @"
+using System.Collections.Generic;
+
+public class TestClass
+{
+    private readonly ExtensionEnumerable values = new();
+
+    public void Test()
+    {
+        foreach (var value in this.values)
+        {
+        }
+    }
+}
+
+public class ExtensionEnumerable
+{
+}
+
+public struct ExtensionEnumerator
+{
+    public int Current => 0;
+
+    public bool MoveNext() => false;
+}
+
+public static class ExtensionEnumerableExtensions
+{
+    public static ExtensionEnumerator GetEnumerator(this ExtensionEnumerable enumerable) => new();
+}
+";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }