mvcc: watcherGroup chooseAll panic (skip proxy tests for now)

[gyuho]

This is the new test #9745 that I've added to simulate #9281 with real-world use case.

Only fails in proxy tests...

https://jenkins-etcd.prod.coreos.systems/job/etcd-proxy/4824/console

=== RUN   TestV3WatchRestoreSnapshotUnsync
panic: watcher current revision should not exceed current revision

goroutine 170309 [running]:
github.com/coreos/etcd/mvcc.(*watcherGroup).chooseAll(0xc4242791c0, 0x10, 0xffffffffffffffff, 0x1)
	/go/src/github.com/coreos/etcd/mvcc/watcher_group.go:241 +0x273
github.com/coreos/etcd/mvcc.(*watcherGroup).choose(0xc4242791c0, 0x200, 0x10, 0xffffffffffffffff, 0xc426479858, 0xc426479858)
	/go/src/github.com/coreos/etcd/mvcc/watcher_group.go:224 +0x289
github.com/coreos/etcd/mvcc.(*watchableStore).syncWatchers(0xc424279180, 0x0)
	/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:340 +0x237
github.com/coreos/etcd/mvcc.(*watchableStore).syncWatchersLoop(0xc424279180)
	/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:214 +0x280
created by github.com/coreos/etcd/mvcc.newWatchableStore
	/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:90 +0x477
FAIL	github.com/coreos/etcd/integration	526.777s

Will investigate in a separate PR.