Add EquivalenceTest

This test asserts that readResolve() works and equivalence ends up with
singletons.

Signed-off-by: Robert Varga <robert.varga@pantheon.tech>

Author: rovarga

triemap/src/test/java/tech/pantheon/triemap/EquivalenceTest.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2024 PANTHEON.tech, s.r.o. and others.  All rights reserved.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v1.0 which accompanies this distribution,
+ * and is available at http://www.eclipse.org/legal/epl-v10.html
+ */
+package tech.pantheon.triemap;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.util.HexFormat;
+import org.junit.jupiter.api.Test;
+
+class EquivalenceTest {
+    @Test
+    void readResolveWorks() throws Exception {
+        final var baos = new ByteArrayOutputStream();
+        try (var oos = new ObjectOutputStream(baos)) {
+            oos.writeObject(Equivalence.Equals.INSTANCE);
+        }
+
+        final var bytes = baos.toByteArray();
+        assertEquals("""
+            aced000573720028746563682e70616e7468656f6e2e747269656d61702e4571756976616c656e636524457175616c7300000000000\
+            0000102000078720021746563682e70616e7468656f6e2e747269656d61702e4571756976616c656e63650000000000000001020000\
+            7870""", HexFormat.of().formatHex(bytes));
+
+        try (var ois = new ObjectInputStream(new ByteArrayInputStream(bytes))) {
+            assertSame(Equivalence.Equals.INSTANCE, ois.readObject());
+        }
+    }
+}