integration: wait on leader before progress check in TestRestartMember

In rare cases, the last member may not have the leader by the time the
final cluster progress check tries to open a watch, causing a timeout.

integration/member_test.go

@@ -61,7 +61,7 @@ func TestRestartMember(t *testing.T) {
 			t.Fatal(err)
 		}
 	}
-
+	c.waitLeader(t, c.Members)
 	clusterMustProgress(t, c.Members)
 }