flaky test: TestEmptyUpdate

[galargh]

I've noticed TestEmptyUpdate to be flaky.

It failed on different OSes in the first two runs here: https://github.com/libp2p/go-smart-record/actions/runs/1564125628/attempts/1

Here's an example of the output from a failed run:

=== RUN   TestEmptyUpdate
      protocol_test.go:121: TTL was not set successfully. The record didn't expire. map[12D3KooWHHzSeKaY8xuZVzkLbKFfvNgPPeKhFBGrMbNzbm5akpqu:0xc000266be8]
  --- FAIL: TestEmptyUpdate (3.02s)

[galargh]

I adjusted the sleep time in the test which seems to have helped - f10e4cc