Use kotlin.test.Test instead of org.junit.Test in common stdlib tests

(and in jvm-only and js-only tests also)

libraries/stdlib/test/concurrent/ThreadTest.kt

@@ -4,7 +4,6 @@ package test.concurrent
 import kotlin.concurrent.*
 import kotlin.test.*
 
-import org.junit.Test
 
 import java.util.concurrent.*
 import java.util.concurrent.TimeUnit.*

libraries/stdlib/test/concurrent/TimerTest.kt

@@ -7,7 +7,6 @@ import kotlin.test.*
 import java.util.concurrent.atomic.AtomicInteger
 import java.util.Timer
 
-import org.junit.Test
 
 class TimerTest {
     @Test fun scheduledTask() {