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

compiler/testData/codegen/controlStructures/ifInWhile.kt

@@ -0,0 +1,14 @@
+import java.lang.Runtime
+
+fun box() : String {
+    val processors = Runtime.getRuntime()!!.availableProcessors()
+    var threadNum = 1
+    while(threadNum <= 1024) {
+        System.out?.println(threadNum)
+        if(threadNum < 2 * processors)
+            threadNum += 1
+        else
+            threadNum *= 2
+    }
+    return "OK"
+}