Fix unchecked runtime shutdown (#4575)

kotlin-native/backend.native/tests/interop/leakMemoryWithRunningThread/checked.kt

@@ -0,0 +1,23 @@
+import leakMemory.*
+import kotlin.native.concurrent.*
+import kotlin.test.*
+import kotlinx.cinterop.*
+
+val global = AtomicInt(0)
+
+fun ensureInititalized() {
+    kotlin.native.initRuntimeIfNeeded()
+    // Leak memory
+    StableRef.create(Any())
+    global.value = 1
+}
+
+fun main() {
+    kotlin.native.internal.Debugging.forceCheckedShutdown = true
+    assertTrue(global.value == 0)
+    // Created a thread, made sure Kotlin is initialized there.
+    test_RunInNewThread(staticCFunction(::ensureInititalized))
+    assertTrue(global.value == 1)
+    // Now exiting. With checked shutdown we will fail, complaining there're
+    // unfinished threads with runtimes.
+}