Fix testAndroidExtensionsManyVariants not using experimental flag

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/AndroidExtensionsManyVariants/app/build.gradle

@@ -34,6 +34,10 @@ android {
     }
 }
 
+androidExtensions {
+    experimental = true
+}
+
 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"