go/format: move example to external test file

Per #11257 all examples should be in external test files.

Additionally, doing so makes this example playable.

Updates #24352. (Albeit tangentially).

Change-Id: I77ab4655107f61db2e9d21a608b73ace3a230fb2
Reviewed-on: https://go-review.googlesource.com/101285Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/format/example_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"log"
+)
+
+func ExampleNode() {
+	const expr = "(6+2*3)/4"
+
+	// parser.ParseExpr parses the argument and returns the
+	// corresponding ast.Node.
+	node, err := parser.ParseExpr(expr)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Create a FileSet for node. Since the node does not come
+	// from a real source file, fset will be empty.
+	fset := token.NewFileSet()
+
+	var buf bytes.Buffer
+	err = format.Node(&buf, fset, node)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println(buf.String())
+
+	// Output: (6 + 2*3) / 4
+}

src/go/format/format_test.go

@@ -6,11 +6,9 @@ package format
 
 import (
 	"bytes"
-	"fmt"
 	"go/parser"
 	"go/token"
 	"io/ioutil"
-	"log"
 	"strings"
 	"testing"
 )
@@ -145,28 +143,3 @@ func TestPartial(t *testing.T) {
 		}
 	}
 }
-
-func ExampleNode() {
-	const expr = "(6+2*3)/4"
-
-	// parser.ParseExpr parses the argument and returns the
-	// corresponding ast.Node.
-	node, err := parser.ParseExpr(expr)
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	// Create a FileSet for node. Since the node does not come
-	// from a real source file, fset will be empty.
-	fset := token.NewFileSet()
-
-	var buf bytes.Buffer
-	err = Node(&buf, fset, node)
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	fmt.Println(buf.String())
-
-	// Output: (6 + 2*3) / 4
-}