[Test] add test for common module

[slievrly]

Why you need it?

Increase the unit test coverage target to 70% +

https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/io/FileLoader.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/holder/ObjectHolder.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/loader/EnhancedServiceLoader.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/loader/ExtensionDefinition.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/StringUtils.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/SizeUtil.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/ReflectionUtil.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/MapUtil.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/LowerCaseLinkHashMap.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/ConfigTools.java
https://app.codecov.io/gh/seata/seata/blob/2.x/common/src/main/java/io/seata/common/util/ArrayUtils.java

contributors claiming this task are requested to submit at least 3 test classes to a pr
请认领此任务的贡献者至少保证每个pr包含3个测试类达到要求。

contributors who plan to claim please comment the test class in issue
计划认领测试任务的贡献者请回复要测试的类。

How it could be?

A clear and concise description of what you want to happen. You can explain more about input of the feature, and output of it.

Other related information

Add any other context or screenshots about the feature request here.

[liuqiufeng]

io.seata.common.io.FileLoader
io.seata.common.holder.ObjectHolder
io.seata.common.util.StringUtils

[liuqiufeng]

io.seata.common.util.ArrayUtils
io.seata.common.util.ConfigTools
io.seata.common.util.MapUtil