metamorphic: use require.Equal in TestParserRandom

Previously the TestParserRandom unit test used pretty.Diff, but the output for
versions with cyclic references has changed. This commit changes the test to
use require.Equal. This requires updating some generated operations to use nil
to represent empty slices rather than []T{}. This matches the ops the parser
creates, which alway use nil for empty slices.

metamorphic/generator.go

@@ -400,6 +400,9 @@ func (g *generator) randValue(min, max int) []byte {
 	if max > min {
 		n += g.rng.Intn(max - min)
 	}
+	if n == 0 {
+		return nil
+	}
 	buf := make([]byte, n)
 	g.fillRand(buf)
 	return buf
@@ -522,10 +525,13 @@ func (g *generator) dbCheckpoint() {
 	// 1/4 of the time we restrict to 1 span;
 	// 1/8 of the time we restrict to 2 spans; etc.
 	numSpans := 0
+	var spans []pebble.CheckpointSpan
 	for g.rng.Intn(2) == 0 {
 		numSpans++
 	}
-	spans := make([]pebble.CheckpointSpan, numSpans)
+	if numSpans > 0 {
+		spans = make([]pebble.CheckpointSpan, numSpans)
+	}
 	for i := range spans {
 		start := g.randKeyToRead(0.01)
 		end := g.randKeyToRead(0.01)

metamorphic/parser_test.go

@@ -6,12 +6,10 @@ package metamorphic
 
 import (
 	"fmt"
-	"strings"
 	"testing"
 
 	"github.com/cockroachdb/datadriven"
 	"github.com/cockroachdb/pebble/internal/randvar"
-	"github.com/kr/pretty"
 	"github.com/stretchr/testify/require"
 )
 
@@ -38,10 +36,7 @@ func TestParserRandom(t *testing.T) {
 	if err != nil {
 		t.Fatalf("%s\n%s", err, src)
 	}
-
-	if diff := pretty.Diff(ops, parsedOps); diff != nil {
-		t.Fatalf("%s\n%s", strings.Join(diff, "\n"), src)
-	}
+	require.Equal(t, ops, parsedOps)
 }
 
 func TestParserNilBounds(t *testing.T) {