Upgrade TestNG to v7.6.1

Currently, we use [TestNG 6.14.2](https://mvnrepository.com/artifact/org.testng/testng/6.14.2) for some of our testing. This version was released in 2018. We recently noticed a bug with `priority` annotation by using TestNG 6.14.2. (See https://github.com/eclipse-openj9/openj9/issues/16284). This issue is resolved in the latest version [TestNG 7.6.1 ](https://mvnrepository.com/artifact/org.testng/testng/7.6.1). Let's upgrade TestNG to use the latest version v7.6.1.

According to my preliminary testing, TestNG 7.6.1 needs another jar - slf4j-api-2.0.3.jar. Not sure if any other jars are needed.

https://github.com/qos-ch/slf4j
https://repo1.maven.org/maven2/org/slf4j/

Start by locating the references to TestNG 6.14.2 and the tests that use the priority annotation. Check the dependency configuration and verify whether slf4j-api-2.0.3.jar or other jars are required, then run the affected tests with TestNG 7.6.1 and confirm they pass.

Written by the indexing model from the issue text.