Kotlin: Add async-await dataflow test case

Author: tamasvajk

java/ql/test/kotlin/library-tests/dataflow/func/coroutine_async_await.kt

@@ -0,0 +1,8 @@
+import kotlinx.coroutines.*
+
+suspend fun fn() {
+    GlobalScope.launch {
+        val x: Deferred<String> = async { Helper.taint() }
+        Helper.sink(x.await())                                  // TODO: not found
+    }
+}

java/ql/test/kotlin/library-tests/dataflow/func/kotlinx_coroutines_stubs.kt

@@ -0,0 +1,33 @@
+/**
+ * Stubs for `kotlinx.coroutines`
+ */
+
+@file:JvmName("BuildersKt") // Required for `async`
+
+package kotlinx.coroutines
+
+public interface CoroutineScope
+public interface CoroutineContext
+public enum class CoroutineStart { DEFAULT }
+public interface Job
+public interface Deferred<out T> : Job {
+    public suspend fun await(): T
+}
+
+public object GlobalScope : CoroutineScope
+
+public fun CoroutineScope.launch(
+    context: CoroutineContext = null!!,
+    start: CoroutineStart = CoroutineStart.DEFAULT,
+    block: suspend CoroutineScope.() -> Unit
+): Job {
+    return null!!
+}
+
+public fun <T> CoroutineScope.async(
+    context: CoroutineContext = null!!,
+    start: CoroutineStart = CoroutineStart.DEFAULT,
+    block: suspend CoroutineScope.() -> T
+): Deferred<T> {
+    return null!!
+}