treewide: use a single append to concatenate two slices

Change-Id: I3930313203c753aa5e629af0db3bf604baaa6b43
Reviewed-on: https://review.monogon.dev/c/monogon/+/3011
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/pkg/logtree/logtree_test.go b/metropolis/pkg/logtree/logtree_test.go
index e37893a..54eabb7 100644
--- a/metropolis/pkg/logtree/logtree_test.go
+++ b/metropolis/pkg/logtree/logtree_test.go
@@ -66,9 +66,7 @@
 
 	var lines []string
 	for _, e := range res.Backlog {
-		for _, msg := range e.Leveled.Messages() {
-			lines = append(lines, msg)
-		}
+		lines = append(lines, e.Leveled.Messages()...)
 	}
 	return lines
 }