Shuffle reports before merging, in benchmark

So that we have equal chance of merging older into newer or vice-versa

app/benchmark_internal_test.go

@@ -2,6 +2,7 @@ package app
 
 import (
 	"flag"
+	"math/rand"
 	"net/http"
 	"net/url"
 	"os"
@@ -65,6 +66,9 @@ func BenchmarkReportUpgrade(b *testing.B) {
 
 func BenchmarkReportMerge(b *testing.B) {
 	reports := upgradeReports(readReportFiles(b, *benchReportPath))
+	rand.Shuffle(len(reports), func(i, j int) {
+		reports[i], reports[j] = reports[j], reports[i]
+	})
 	merger := NewFastMerger()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {