clientv3/integration: fix race in TestWatchCompactRevision

[xiang90]

Fix #5982

Reduce the cluster size to 1 for this test. It is not necessary to have a 3 member cluster for this test at the first place.

If we use a 3 member cluster, the watch request might arrive before the compaction propagates to that member. Then watch with rev 2 will succeed.

[heyitsanthony]

@xiang90 I recall the intent for 3 members was to ensure the compact propagated. I think the fix would be a linearized read before the watch?

[xiang90]

@heyitsanthony This is a test for client, not for etcd server. I feel we should not test propagation here.

[heyitsanthony]

ok lgtm