Vendor dependencies for GCS

vendor/cloud.google.com/go/internal/readme/testdata/want.md

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+This template is for testing snipmd.awk.
+
+Put the first snippet here.
+
+[snip]:# (first)
+```go
+fmt.Println("hello")
+x := ...
+```
+
+And now the second.
+[snip]:# (second)
+```go
+if x > 2 {
+	...
+}
+```
+
+A top-level snippet.
+
+[snip]:# (top-level)
+```go
+var ErrBad = errors.New("bad")
+```
+
+```go
+// A code block that is not included.
+```
+
+And we're done.