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/io/FileTreeWalks.kt

@@ -1,7 +1,6 @@
 @file:kotlin.jvm.JvmVersion
 package test.io
 
-import org.junit.Test
 import java.io.File
 import java.io.IOException
 import java.util.*

libraries/stdlib/test/io/Files.kt

@@ -3,7 +3,6 @@ package test.io
 
 import java.io.*
 import java.util.*
-import org.junit.Test
 import kotlin.io.walkTopDown
 import kotlin.test.*
 

libraries/stdlib/test/io/IOStreams.kt

@@ -1,7 +1,6 @@
 @file:kotlin.jvm.JvmVersion
 package test.io
 
-import org.junit.Test
 import kotlin.test.*
 import java.io.Writer
 import java.io.BufferedReader

libraries/stdlib/test/io/ReadWrite.kt

@@ -1,15 +1,12 @@
 @file:kotlin.jvm.JvmVersion
 package test.io
 
-import org.junit.Test
+import kotlin.test.*
 import java.io.File
-import kotlin.test.assertEquals
 import java.io.Reader
 import java.io.StringReader
 import java.net.URL
 import java.util.ArrayList
-import kotlin.test.assertFalse
-import kotlin.test.assertTrue
 
 fun sample(): Reader = StringReader("Hello\nWorld");
 

libraries/stdlib/test/io/SerializableTest.kt

@@ -2,7 +2,6 @@
 package test.io
 
 import java.io.*
-import org.junit.Test
 import kotlin.test.*
 
 private class Serial(val name: String) : Serializable {