Update TestNG (#228)

The used version triggers vulnerability, but the dependency
is here only as optional to provide support for tests by
using TestNG annotations.

Author: cstamas

pom.xml

@@ -269,7 +269,7 @@
       <dependency>
         <groupId>org.testng</groupId>
         <artifactId>testng</artifactId>
-        <version>6.14.3</version>
+        <version>7.5.1</version>
       </dependency>
       <dependency>
         <groupId>org.apache.felix</groupId>