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Starting from build NetBeans 201204030400, testing Maven project will not automatically open Test Results window. Because that is working for Java SE and because this is part of JUnit 72 TS, I marked it as P2.
Updated Target Milestone
I am sorry but I cannot reproduce in NetBeans IDE Dev (Build 20120404-19a1cb26d752). Both JavaSE and maven java project have the same behavior. If all tests pass the Test Result Window does not pop-up. If at least one test fails the TRW opens. You can see bug 210554 for further discussion on this.