Add -update flag that can be used to update golden files. (#467)

This is similar to such flag in driver_test.go and dotgraph_test.go.

I had this change locally for some time, would like to check this in.

profile/profile_test.go

 
 import (
 	"bytes"
+	"flag"
 	"fmt"
 	"io/ioutil"
 	"path/filepath"
 	"github.com/google/pprof/internal/proftest"
 )
 
+var update = flag.Bool("update", false, "Update the golden files")
+
 func TestParse(t *testing.T) {
 	const path = "testdata/"
 
 
 		js := p.String()
 		goldFilename := path + source + ".string"
+		if *update {
+			err := ioutil.WriteFile(goldFilename, []byte(js), 0644)
+			if err != nil {
+				t.Errorf("failed to update the golden file file %q: %v", goldFilename, err)
+			}
+		}
 		gold, err := ioutil.ReadFile(goldFilename)
 		if err != nil {
 			t.Fatalf("%s: %v", source, err)