Fix nondeterministic behavior in testConcurrentHashMapKeySetViewCopy()

[annhchen89]

What does this PR do?

This PR fixes a nondeterministic test failure in DefaultSerializersTest.testConcurrentHashMapKeySetViewCopy() within both the main and main-versioned modules when running with NonDex.

Problem

The nondeterminism arises from using assertEquals(set.iterator().next(), copy.iterator().next()), which depends on the iteration order of a ConcurrentHashMap.KeySetView.
Since ConcurrentHashMap does not guarantee a stable iteration order, the first element returned by iterator().next() can vary between runs, causing the test to fail under NonDex.

Reproduce Test

To reproduce the failure, run NonDex on both the main and main-versioned modules using the following commands:

mvn -pl main edu.illinois:nondex-maven-plugin:2.2.1:nondex \
  -Dtest=com.esotericsoftware.kryo.serializers.DefaultSerializersTest#testConcurrentHashMapKeySetViewCopy

mvn -pl main-versioned edu.illinois:nondex-maven-plugin:2.2.1:nondex \
  -Dtest=com.esotericsoftware.kryo.serializers.DefaultSerializersTest#testConcurrentHashMapKeySetViewCopy

The Fix

Instead of comparing the first element returned by each iterator — which could differ due to the nondeterministic iteration order of ConcurrentHashMap — the test now verifies that both collections contain the same elements.
This approach ensures logical equality without relying on iteration order, effectively confirming that both sets represent the same data regardless of internal ordering.