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

compiler/testData/codegen/controlStructures/tryCatch.kt

@@ -0,0 +1,9 @@
+fun foo(s: String): String? {
+  try {
+    Integer.parseInt(s);
+    return "no message";
+  }
+  catch(e: NumberFormatException) {
+    return (e : Throwable).getMessage(); // Work around an overload-resolution bug
+  }
+}