Renamed *.jet to *.kt in test data of JetPsiCheckerTest.

idea/testData/checker/regression/Jet121.kt

@@ -0,0 +1,13 @@
+package jet121
+  fun box() : String {
+    val answer = apply("OK") { String.() : Int ->
+      get(0)
+      length
+    }
+
+    return if (answer == 2) "OK" else "FAIL"
+  }
+
+  fun apply(arg:String, f :  String.() -> Int) : Int {
+      return arg.f()
+  }