eth TestCheckpointChallenge fails frequently in Travis builds

[adamschmideg]

This is how it fails here

TestCheckpointChallenge/sync_fast_checkpoint_true_timeout_true_empty_false_match_true
    handler_test.go:542: peer count mismatch: have 1, want 0

[rjl493456442]

After running stress 756 runs so far, 0 failures no failure actually occurs.

Though we can blindly increase the time sleep duration to reduce the possibility of failure

[adamschmideg]

It hasn't shown up in the last 2 weeks