Use separate TestKitDir per project to support parallel daemons

Without this running in parallel is likely to get you an error from
Gradle timing out waiting to lock the jars directory:
"It is currently in use by another Gradle instance."

Author: deepy

src/test/groovy/com/github/gradle/AbstractIntegTest.groovy

@@ -36,6 +36,7 @@ abstract class AbstractIntegTest extends Specification {
                 .withPluginClasspath()
                 .forwardOutput()
                 .withGradleVersion(gradleVersion.version)
+                .withTestKitDir(createFile(".gradle-homedir"))
                 .withDebug(false)
     }