cmd/go: run 'go help documentation' through gofmt in TestDocsUpToDate

mkalldocs.sh runs gofmt on the output, but the test does not.
If go help documentation and gofmt disagree, the test will fail.
Fix that.

Change-Id: I837374a2d36cb5d71278ecefe2a7b6544622c576
Reviewed-on: https://go-review.googlesource.com/c/go/+/384256
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/cmd/go/help_test.go

@@ -6,6 +6,8 @@ package main_test
 
 import (
 	"bytes"
+	"go/format"
+	diffpkg "internal/diff"
 	"os"
 	"testing"
 
@@ -23,11 +25,17 @@ func TestDocsUpToDate(t *testing.T) {
 	buf := new(bytes.Buffer)
 	// Match the command in mkalldocs.sh that generates alldocs.go.
 	help.Help(buf, []string{"documentation"})
-	data, err := os.ReadFile("alldocs.go")
+	internal := buf.Bytes()
+	internal, err := format.Source(internal)
+	if err != nil {
+		t.Fatalf("gofmt docs: %v", err)
+	}
+	alldocs, err := os.ReadFile("alldocs.go")
 	if err != nil {
 		t.Fatalf("error reading alldocs.go: %v", err)
 	}
-	if !bytes.Equal(data, buf.Bytes()) {
-		t.Errorf("alldocs.go is not up to date; run mkalldocs.sh to regenerate it")
+	if !bytes.Equal(internal, alldocs) {
+		t.Errorf("alldocs.go is not up to date; run mkalldocs.sh to regenerate it\n%s",
+			diffpkg.Diff("go help documentation | gofmt", internal, "alldocs.go", alldocs))
 	}
 }