By "more object-oriented", I mean, it appears to me testing frameworks like TestNG and JUnit could encourage testers to write implementations of Test and TestSuite interfaces. The current approach using @Testannotations often leads to huge classes with lots of methods and lots of boilerplate code.

A more "OO" way could, for example, look like this:

public class MyTestSuite extends framework.AbstractTestSuite {

  @Override
  public void setup() {
    //...
  }

  @Override
  public void execution() {
    executeInParallel(
      new MyTestA("some param"), 
      new MyTestA("some other param"),
      new MyTestB());
    execute(new MyTestBSubclass());
  }

  @Override
  public void teardown() {
    //...
  }
}

Are there problems with this approach I'm not anticipating?