(test data) Revert removal of coroutines opt-in from a test

This test checks the coroutines opt-in, so it should stay there.

Revert the change made in a64a76d5

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

@@ -15,6 +15,12 @@ repositories {
     mavenCentral()
 }
 
+kotlin {
+    experimental {
+        coroutines 'enable'
+    }
+}
+
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 }