To avoid any network use unless necessary, delay establishing authorization

challenges with the upstream until any proxied data is found not to be local.

Implement auth challenges behind an interface and add to unit tests.  Also,
remove a non-sensical unit test.

Signed-off-by: Richard Scothern <richard.scothern@docker.com>

registry/proxy/proxyblobstore_test.go

@@ -168,6 +168,7 @@ func makeTestEnv(t *testing.T, name string) *testEnv {
 		remoteStore:    truthBlobs,
 		localStore:     localBlobs,
 		scheduler:      s,
+		authChallenger: &mockChallenger{},
 	}
 
 	te := &testEnv{
@@ -242,6 +243,11 @@ func TestProxyStoreStat(t *testing.T) {
 	if (*remoteStats)["stat"] != remoteBlobCount {
 		t.Errorf("Unexpected remote stat count")
 	}
+
+	if te.store.authChallenger.(*mockChallenger).count != len(te.inRemote) {
+		t.Fatalf("Unexpected auth challenge count, got %#v", te.store.authChallenger)
+	}
+
 }
 
 func TestProxyStoreServeHighConcurrency(t *testing.T) {