This fixes a flake I've seen in the wild lately:
```
--- FAIL: TestBlockDifferenceFix (0.01s)
router_test.go:4335: height should have been updated to 5, instead got 4
FAIL
FAIL github.com/lightningnetwork/lnd/routing 3.865s
FAIL
```
We wrap things in an assertion loop to ensure that timing quirks don't
cause the test to fail sporadically.