We don't consistently reach the same etcd server during the lifetime of
a test and in some cases, this means that this test will flake if an
etcd server was slow to update its state and the test hits the outdated
server.
Here we switch to using an `Eventually` case which will wait upto a
second for the expected result before failing - with a 10ms gap between
invocations.
```
[tests(dani/leasefix)] $ gotestsum -- ./common -tags integration -count 100 -timeout 15m -run TestLeaseGrantAndList
✓ common (2m26.71s)
DONE 1600 tests in 147.258s
```
