Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/controlStructures/sync.kt

@@ -0,0 +1,33 @@
+import java.util.concurrent.*
+import java.util.concurrent.atomic.*
+
+fun thread(block: ()->Unit ) {
+    val thread = object: Thread() {
+        override fun run() {
+            block()
+        }
+    }
+    thread.start()
+}
+
+fun box() : String {
+   val mtref = AtomicInteger()
+   val cdl = CountDownLatch(11)
+   for(i in 0..10) {
+       thread {
+          var current = 0
+          do {
+              current = synchronized(mtref) {
+                val v = mtref.get() + 1
+                if(v < 100)
+                    mtref.set(v+1)
+                v
+              }
+          }
+          while(current < 100)
+          cdl.countDown()
+       }
+   }
+   cdl.await()
+   return if(mtref.get() == 100) "OK" else mtref.get().toString()
+}