token-server: Fix panic when generating machine token for unrecognized machine.

tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go

Index: tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go
diff --git a/tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go b/tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go
index cb45ea61c3b0806a053509d574cdc02f0f512764..f48f4a84932f12d55fae2974732fc71b884e9ce0 100644
--- a/tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go
+++ b/tokenserver/appengine/impl/machinetoken/rpc_mocks_test.go
@@ -109,14 +109,13 @@ var testingCA = certconfig.CA{
 	UpdatedRev: "cfg-updated-rev",
 }
 
-func testingContext() context.Context {
+func testingContext(ca certconfig.CA) context.Context {
 	ctx := gaetesting.TestingContext()
 	ctx = info.GetTestable(ctx).SetRequestID("gae-request-id")
 	ctx, _ = testclock.UseTime(ctx, testingTime)
 
 	// Put mocked CA config in the datastore.
-	ca := testingCA
-	ds.Put(ctx, &ca) // put a copy, Put modifies the object
+	ds.Put(ctx, &ca)
 	certconfig.StoreCAUniqueIDToCNMap(ctx, map[int64]string{
 		ca.ParsedConfig.UniqueId: ca.CN,
 	})