Balancer tests have been flaky since last balancer rewrite.
Should be from slow balancer failover. Once we re-rewrite balancer
with roundrobin, there should be no more flaky tests as below:
TestBalancerUnderServerStopInflightLinearizableGetOnRestart (27.62s)
server_shutdown_test.go:343: context deadline exceeded
TestBalancerUnderServerShutdownDelete (3.22s)
util.go:32: context deadline exceeded
Signed-off-by: Gyuho Lee <gyuhox@gmail.com>