goose: add goose.Value directive

Subsumes previous usage of goose.Optional.

Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/VarValue/foo/foo.go

@@ -0,0 +1,26 @@
+// Copyright 2018 Google LLC
+//
+// 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
+//
+//     https://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 main
+
+import "fmt"
+
+func main() {
+	// Value should be deferred until function call.
+	msg = "Hello, World!"
+
+	fmt.Println(injectedMessage())
+}
+
+var msg string