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

compiler/testData/codegen/controlStructures/forIntRange.kt

@@ -0,0 +1,13 @@
+fun box() : String {
+    val a = arrayOfNulls<String>(3)
+    a[0] = "a"
+    a[1] = "b"
+    a[2] = "c"
+
+    var result = 0
+    for(i in a.indices) {
+      result += i
+    }
+    if (result != 3) return "FAIL"
+    return "OK"
+}