Unit test TestWorkerMasterFailover is not stable

[amyangfei]

ref: https://github.com/hanfei1991/microcosm/runs/6204735573?check_suite_focus=true
from pr: #318, commit: 5b70fd6

--- FAIL: TestWorkerMasterFailover (1.01s)
    worker_test.go:193: 
        	Error Trace:	worker_test.go:193
        	Error:      	Condition never satisfied
        	Test:       	TestWorkerMasterFailover
FAIL
coverage: 65.7% of statements
FAIL	github.com/hanfei1991/microcosm/lib	2.842s