Add test cases for nullable Equals methods

Adds test cases for Equals methods with nullable reference type parameters
(object?) to verify the fix works correctly.

Co-authored-by: hvitved <3667920+hvitved@users.noreply.github.com>

Author: Copilot

csharp/ql/test/library-tests/frameworks/system/Equals/Equals.cs

@@ -24,3 +24,16 @@ struct Equals1Struct
 {
     public override bool Equals(object other) => false;
 }
+
+#nullable enable
+
+class NullableEquals1
+{
+    public override bool Equals(object? other) => false;
+}
+
+class NullableEquals2 : IEquatable<NullableEquals2>
+{
+    public bool Equals(NullableEquals2? other) => other != null;
+    public override bool Equals(object? other) => other is NullableEquals2 n && Equals(n);
+}

csharp/ql/test/library-tests/frameworks/system/Equals/Equals.expected

@@ -5,3 +5,5 @@
 | Equals.cs:16:7:16:13 | Equals3 | Equals3.Equals(Equals3) | true |
 | Equals.cs:21:8:21:21 | NoEqualsStruct | System.ValueType.Equals(object) | false |
 | Equals.cs:23:8:23:20 | Equals1Struct | Equals1Struct.Equals(object) | true |
+| Equals.cs:31:7:31:21 | NullableEquals1 | NullableEquals1.Equals(object) | true |
+| Equals.cs:36:7:36:21 | NullableEquals2 | NullableEquals2.Equals(NullableEquals2) | true |