testing frameworks should be applied as plugins

as discussed here: http://gradle.1045684.n5.nabble.com/Junit-and-TestNG-tp3073824p3073824.html
testing frameworks (i.e. junit and testng) should be applied as other plugins:

// Adds the main source set and implicitly the standard lifecycle but no test task
apply plugin: 'java'

// Adds in the 'junit' task plus implicitly the test source set
apply plugin: 'junit'

// Adds in the 'testng' task plus implicitly the test source set
apply plugin: 'testng'