[testbox] Go example for closing channels

Summary: Example to show the closing of channels.

Test Plan:
```
[chetan@ip-10-160-42-191 go]$ bin/gocov test examples/closing_channels/ | tee /tmp/gocov_closing_channels | bin/gocov report
warning: building out-of-date packages:
  github.com/axw/gocov
installing these packages with 'go test -i examples/closing_channels/' will speed future tests.

ok    examples/closing_channels 0.007s

examples/closing_channels//closing_channels.go   closingChannels 93.75% (15/16)
examples/closing_channels/       --------------- 93.75% (15/16)

[chetan@ip-10-160-42-191 go]$ bin/gocov annotate /tmp/gocov_closing_channels

 5      func closingChannels() {
 6        jobs := make(chan int)
 7        done := make(chan bool)
 8
 9        go func() {
10          for {
11            j, more := <-jobs
12            if more {
13              fmt.Println("Received job", j)
14            } else {
15              fmt.Println("Received all jobs")
16              done <- true
17 MISS         return
18            }
19          }
20        }()
21
22        for j := 1; j <= 3; j++ {
23          jobs <- j
24          fmt.Println("Sent job", j)
25        }
26
27        close(jobs)
28        fmt.Println("Sent all jobs")
29        <-done
30      }
```

Reviewers: tommy.testy

Reviewed By: tommy.testy

Differential Revision: http://dev.chetangowda.com/D27