De-flake TestJetStreamClusterRedeliverBackoffs (#6331)

`TestJetStreamClusterRedeliverBackoffs` would sometimes fail, even
though it would be very close to the intended backoff:
```
jetstream_cluster_2_test.go:4216: Timing is off for 1, expected ~100ms, but got 99.846642ms
jetstream_cluster_2_test.go:4216: Timing is off for 0, expected ~25ms, but got 50.767748ms
jetstream_cluster_2_test.go:4216: Timing is off for 4, expected ~250ms, but got 249.95211ms
```

Allowing for a bit of leeway on both ends.

Signed-off-by: Maurice van Veen <github@mauricevanveen.com>

server/jetstream_cluster_2_test.go

@@ -4219,7 +4219,7 @@ func TestJetStreamClusterRedeliverBackoffs(t *testing.T) {
 		d := tr.Sub(start)
 		// Adjust start for next calcs.
 		start = start.Add(d)
-		if d < expected[i] || d > expected[i]*2 {
+		if d < expected[i]-5*time.Millisecond || d > expected[i]*2+5*time.Millisecond {
 			t.Fatalf("Timing is off for %d, expected ~%v, but got %v", i, expected[i], d)
 		}
 	}