Add missing package statement to test file (Kotlin#3249)

Author: shmuelr

kotlinx-coroutines-core/concurrent/test/LimitedParallelismConcurrentTest.kt

@@ -2,6 +2,8 @@
  * Copyright 2016-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
+package kotlinx.coroutines
+
 import kotlinx.atomicfu.*
 import kotlinx.coroutines.*
 import kotlinx.coroutines.exceptions.*