Mark redir test target as testonly

Change-Id: I99a71306ac944b7151dfaa9991dae02f18c4e452

third_party/nacl-ported-tests/redir/BUILD.gn

diff --git a/third_party/nacl-ported-tests/redir/BUILD.gn b/third_party/nacl-ported-tests/redir/BUILD.gn
index c6bba27..de75920 100644
--- a/third_party/nacl-ported-tests/redir/BUILD.gn
+++ b/third_party/nacl-ported-tests/redir/BUILD.gn
@@ -3,6 +3,8 @@
 # found in the LICENSE file.
 
 source_set("small_tests") {
+  testonly = true
+
   sources = [
     "redir_test.cc",
   ]