Add tests when can not connect to memcaced server

[h3poteto]

Now tests has error when memcached is not running.

  ● connection › multiple set and get › should get

    Error [ERR_UNHANDLED_ERROR]: Unhandled error. (undefined)

      27 |     return new Promise(resolve => {
      28 |       this.connection = new Connection(this.host, this.port, this.options.timeout)
    > 29 |       this.connection.on('error', () => this.emit('error'))
         |                                              ^
      30 |       this.connection.on('timeout', () => this.emit('timeout'))
      31 |       this.connection.on('connected', () => {
      32 |         this.emit('connected')

      at Connection.<anonymous> (src/memcached.ts:29:46)
      at Socket.<anonymous> (src/connection.ts:61:41)

But this message is not kindness. Please create a test case to confirm memcached server connection.