backend/test: fix delayedRemoval timeout handling

The timeout for all blobs starts to run after the delete calls have been
issue. Thus, use the same start time for all blobs instead of individual
timeouts.

internal/backend/test/tests.go

@@ -740,8 +740,8 @@ func (s *Suite[C]) delayedRemove(t testing.TB, be backend.Backend, handles ...ba
 		return err
 	}
 
+	start := time.Now()
 	for _, h := range handles {
-		start := time.Now()
 		attempt := 0
 		var found bool
 		var err error