Fix a couple of flaky tests (#4478)

* Fix cancellable continuation's toString check

A smart enough compiler can inline a lambda being called
immediately after being defined, which can trigger the tail call
optimization, leading to a test error.

* Fix RunningThreadStackMergeTest flakiness

Ensure that the thread actually calls `park` before proceeding.

Author: dkhalanskyjb

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

@@ -15,7 +15,7 @@ class CancellableContinuationJvmTest : TestBase() {
             it.resume(Unit)
             assertTrue(it.toString().contains("kotlinx.coroutines.CancellableContinuationJvmTest.checkToString(CancellableContinuationJvmTest.kt"))
         }
-        suspend {}() // Eliminate tail-call optimization
+        yield() // Eliminate tail-call optimization
     }
 
     @Test

kotlinx-coroutines-debug/test/RunningThreadStackMergeTest.kt

@@ -9,6 +9,7 @@ import kotlin.test.*
 
 class RunningThreadStackMergeTest : DebugTestBase() {
 
+    private var coroutineThread: Thread? = null
     private val testMainBlocker = CountDownLatch(1) // Test body blocks on it
     private val coroutineBlocker = CyclicBarrier(2) // Launched coroutine blocks on it
 
@@ -39,6 +40,10 @@ class RunningThreadStackMergeTest : DebugTestBase() {
         while (coroutineBlocker.numberWaiting != 1) {
             Thread.sleep(10)
         }
+        // Wait for the coroutine to actually call `LockSupport.park`
+        while (coroutineThread?.state != Thread.State.WAITING) {
+            Thread.sleep(10)
+        }
     }
 
     private fun CoroutineScope.launchCoroutine() {
@@ -59,6 +64,7 @@ class RunningThreadStackMergeTest : DebugTestBase() {
     }
 
     private fun nonSuspendingFun() {
+        coroutineThread = Thread.currentThread()
         testMainBlocker.countDown()
         coroutineBlocker.await()
     }
@@ -67,7 +73,6 @@ class RunningThreadStackMergeTest : DebugTestBase() {
     fun testStackMergeEscapeSuspendMethod() = runTest {
         launchEscapingCoroutine()
         awaitCoroutineStarted()
-        Thread.sleep(10)
         verifyDump(
             "Coroutine \"coroutine#2\":StandaloneCoroutine{Active}@3aea3c67, state: RUNNING\n" +
                 "\tat jdk.internal.misc.Unsafe.park(Native Method)\n" +