address already in use with some server's test.

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

     tidb_test.go:3198: 
        	Error Trace:	server/tidb_test.go:3198
        	Error:      	Received unexpected error:
        	            	listen tcp 127.0.0.1:4000: bind: address already in use
        	            	github.com/pingcap/errors.AddStack
        	            		external/com_github_pingcap_errors/errors.go:174
        	            	github.com/pingcap/errors.Trace
        	            		external/com_github_pingcap_errors/juju_adaptor.go:15
        	            	github.com/pingcap/tidb/server.NewServer
        	            		server/server.go:275
        	            	github.com/pingcap/tidb/server.TestProxyProtocolWithIpNoFallbackable
        	            		server/tidb_test.go:3197
        	            	testing.tRunner
        	            		GOROOT/src/testing/testing.go:1576
        	            	runtime.goexit
        	            		src/runtime/asm_amd64.s:1598
        	Test:       	TestProxyProtocolWithIpNoFallbackable 

4. What is your TiDB version? (Required)