Add License decl to testing.go

Test: Existing tests pass
Change-Id: I0d8b77d87801cb361036c8788f30bd80070fd7ff
diff --git a/bp2build/testing.go b/bp2build/testing.go
index ba328e9..3a77d0e 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -1,3 +1,17 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package bp2build
 
 /*