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

compiler/testData/codegen/controlStructures/forPrimitiveIntArray.kt

@@ -0,0 +1,14 @@
+fun box() : String {
+    val a = IntArray (5)
+    var i = 0
+    var sum = 0
+    for(el in 0..4) {
+       a[i] = i++
+    }
+    for (el in a) {
+        sum = sum + el
+    }
+    if(sum != 10) return "a failed"
+
+    return "OK"
+}