Make a complete copy of Test task

You could probably do

TestFrameworkOptions options = testTask.getOptions();
if (options instanceof TestNGOptions) {
   testTaskcopy.useTestNG();
} else if (options instanceof JUnitOptions) {
   testTaskCopy.useJunit();
} else if (options instanceof JUnitPlatformOptions) {
   testTaskCopy.useJunitPlatform();
}

See TestFrameworkOptions