Fix one possible data race in admin tests (#6537)
Fix one possible data race in admin tests (#6537)

go test shows the following warning:
```
WARNING: DATA RACE
Write at 0x000002909e18 by goroutine 276:
  github.com/minio/minio/cmd.testAdminCmdRunnerSignalService()
      /home/travis/gopath/src/github.com/minio/minio/cmd/admin-rpc_test.go:44 +0x94

 Previous read at 0x000002909e18 by goroutine 194:
  github.com/minio/minio/cmd.testServiceSignalReceiver()
      /home/travis/gopath/src/github.com/minio/minio/cmd/admin-handlers_test.go:467 +0x70
```

The reason for this data race is that some admin tests are not waiting for go routines
that they created to be properly exited, which triggers the race detector.
fwessels/minio,NitishT/minio,fwessels/minio,aead/minio,harshavardhana/minio,aead/minio,aead/minio,harshavardhana/minio,jimjag/minio,harshavardhana/minio,balamurugana/minio,balamurugana/minio,harshavardhana/minio,aead/minio,aead/minio,donatello/minio,krishnasrinivas/minio,NitishT/minio,harshavardhana/minio,donatello/minio,fwessels/minio,jimjag/minio,vadmeste/minio,harshavardhana/minio,krishnasrinivas/minio,krishnasrinivas/minio,harshavardhana/minio,aead/minio,jimjag/minio,minio/minio,harshavardhana/minio,minio/minio,vadmeste/minio,aead/minio,aead/minio,krisis/minio,krishnasrinivas/minio,jimjag/minio,krisis/minio,NitishT/minio,NitishT/minio,fwessels/minio