Fixed visibility of atomic properties, they are required to be private or internal (#3808)

The new atomicfu JVM compiler plugin (since 1.9.20) will explicitly require atomic properties to be private or internal (or to be members of private or internal classes).

See: Kotlin/kotlinx-atomicfu#322
YT issue: KT-60528

Author: mvicsokolova

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

@@ -68,7 +68,7 @@ class LimitedParallelismConcurrentTest : TestBase() {
     @Test
     fun testNotDoingDispatchesWhenNoTasksArePresent() = runTest {
         class NaggingDispatcher: CoroutineDispatcher() {
-            val closed = atomic(false)
+            private val closed = atomic(false)
             override fun dispatch(context: CoroutineContext, block: Runnable) {
                 if (closed.value)
                     fail("Dispatcher was closed, but still dispatched a task")

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

@@ -9,7 +9,7 @@ import kotlin.coroutines.*
 import kotlin.test.*
 
 class MultithreadedDispatcherStressTest {
-    val shared = atomic(0)
+    private val shared = atomic(0)
 
     /**
      * Tests that [newFixedThreadPoolContext] will not drop tasks when closed.

kotlinx-coroutines-core/jvm/test/JobHandlersUpgradeStressTest.kt

@@ -29,7 +29,7 @@ class JobHandlersUpgradeStressTest : TestBase() {
     @Volatile
     private var job: Job? = null
 
-    class State {
+    internal class State {
         val state = atomic(0)
     }