This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
The cost of a Groovy script is starting up the JVM. Once that's done, however, it's as fast as anything else.
You can implement JUnit tests in Groovy, or you can use Spock, which is a testing library that uses Groovy. Check out the source code repo for my book, https://github.com/kousen/Making-Java-Groovy for examples. Chapter 6 is the testing chapter.