Add more unit test for `SingleXAResourceHolder`

[FlyingZC]

Hi community, This issue is for #20305

Aim

Add more unit tests for the class of SingleXAResourceHolder.

Basic Qualifications

• Java
• Maven
• Transaction

Detail

Add more unit tests for org.apache.shardingsphere.transaction.xa.bitronix.manager.SingleXAResourceHolder to test its public functions to improve unit test coverage.

Unit test coverage can be viewed at: https://app.codecov.io/gh/apache/shardingsphere