cmd/doc: fix test on nacl

nacl is really giving a hard time. avoid all external dependencies in the test.
Worked with trybots, failed in the build. No explanation, but this should fix it.

TBR=rsc

Change-Id: Icb644286dbce88f17ee3d96ad90efba34a80a92d
Reviewed-on: https://go-review.googlesource.com/11291Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/doc_test.go

@@ -30,9 +30,9 @@ const p = "cmd/doc/testdata"
 var tests = []test{
 	// Sanity check.
 	{
-		"fmt",
-		[]string{`fmt`},
-		[]string{`type Formatter interface`},
+		"sanity check",
+		[]string{p},
+		[]string{`type ExportedType struct`},
 		nil,
 	},