Work around is a simple one: adding the following line to the test task

binResultsDir = file("$buildDir/binary-results/")

But that causes a warning if gradle version is 1.3 or below.

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I don't see this behavior anymore with Gradle 3.3 for the following example build script:

apply plugin: 'groovy'

sourceSets {
    integrationTest {
        groovy.srcDir file('src/integTest/groovy')
        resources.srcDir file('src/integTest/resources')
        compileClasspath += sourceSets.main.output + configurations.testRuntime
        runtimeClasspath += output + compileClasspath
    }
}

task integrationTest(type: Test) {
    testClassesDir = sourceSets.integrationTest.output.classesDir
    classpath = sourceSets.integrationTest.runtimeClasspath
}