testutil: whitelist os.(*file).close

[heyitsanthony]

Leak detector is catching goroutines trying to close files which appear
runtime related:

1 instances of:
syscall.Syscall(...)
/usr/local/golang/1.8.3/go/src/syscall/asm_linux_386.s:20 +0x5
syscall.Close(...)
/usr/local/golang/1.8.3/go/src/syscall/zsyscall_linux_386.go:296 +0x3d
os.(*file).close(...)
/usr/local/golang/1.8.3/go/src/os/file_unix.go:140 +0x62

It's unlikely a user goroutine will leak on file close; whitelist it.

[gyuho]

lgtm. thanks

[codecov-io]

Codecov Report

Merging #8284 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #8284      +/-   ##
=========================================
- Coverage   76.42%   76.3%   -0.13%     
=========================================
  Files         346     346              
  Lines       27055   27056       +1     
=========================================
- Hits        20676   20644      -32     
- Misses       4901    4926      +25     
- Partials     1478    1486       +8

Impacted Files	Coverage Δ
pkg/testutil/leak.go	67.64% <100%> (+0.48%)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
pkg/adt/interval_tree.go	85.88% <0%> (-5.11%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
rafthttp/msgappv2_codec.go	67.82% <0%> (-3.48%)	⬇️
clientv3/concurrency/election.go	80% <0%> (-2.41%)	⬇️
pkg/netutil/netutil.go	62.35% <0%> (-2.36%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
etcdmain/etcd.go	45.06% <0%> (-1.29%)	⬇️
clientv3/watch.go	94.5% <0%> (-1%)	⬇️
... and 12 more

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