storage/app: skip tests on plan9

The whole package was alread marked as +build !plan9, but I missed the
tests before.

Change-Id: I1f80512e8fbe939709d7159d9ab67c061f3433ba
Reviewed-on: https://go-review.googlesource.com/87256
Reviewed-by: David du Colombier <[email protected]>

Author: bradfitz

storage/app/query_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !plan9
+
 package app
 
 import (