How to change JDK of JUnitTestSuiteSpec


I have some junit tests that only run under java8. Before using the software model I could change the test task so that it would use java8 by doing

test {
  sourceCompatibility = 1.8
  targetCompatibility = 1.8

But after converting to use the software model the build fails cause it is trying to compile the tests using java 7

Is there a way to have it test all the different target java platforms but always compile the tests using java 8?