At the moment, Azure DevOps supports the following results formats include CTest, JUnit (including PHPUnit), NUnit 2, NUnit 3, Visual Studio Test (TRX), and xUnit 2. Unfortunately, it does not support TESTNG test results format. The good news, however, is that TESTNG also produces JUnit test results in a separate junitreports folder. So, we can publish TESTNG test results in JUnit format instead.
To do that, just change Test results files field under JUnit Test Results section to **/junitreports/TEST-*.xml.