With these not being closed, they were continuing to consume resources
after the return of the test function, which was affecting the later
BenchmarkSendQueue.

Before:
```
snowflake/common/turbotunnel$ go test -bench BenchmarkSendQueue -v
=== RUN   TestQueueIncomingOversize
--- PASS: TestQueueIncomingOversize (0.00s)
=== RUN   TestWriteToOversize
--- PASS: TestWriteToOversize (0.00s)
=== RUN   TestRestoreMTU
--- PASS: TestRestoreMTU (0.00s)
=== RUN   TestRestoreCap
--- PASS: TestRestoreCap (0.00s)
=== RUN   TestQueuePacketConnWriteToKCP
--- PASS: TestQueuePacketConnWriteToKCP (1.01s)
goos: linux
goarch: amd64
pkg: gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/turbotunnel
cpu: Intel(R) Core(TM) i5 CPU         680  @ 3.60GHz
BenchmarkSendQueue
BenchmarkSendQueue-4     8519708               136.0 ns/op
PASS
ok      gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/turbotunnel  3.481s
```

After:
```
snowflake/common/turbotunnel$ go test -bench BenchmarkSendQueue -v
=== RUN   TestQueueIncomingOversize
--- PASS: TestQueueIncomingOversize (0.00s)
=== RUN   TestWriteToOversize
--- PASS: TestWriteToOversize (0.00s)
=== RUN   TestRestoreMTU
--- PASS: TestRestoreMTU (0.00s)
=== RUN   TestRestoreCap
--- PASS: TestRestoreCap (0.00s)
=== RUN   TestQueuePacketConnWriteToKCP
--- PASS: TestQueuePacketConnWriteToKCP (1.02s)
goos: linux
goarch: amd64
pkg: gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/turbotunnel
cpu: Intel(R) Core(TM) i5 CPU         680  @ 3.60GHz
BenchmarkSendQueue
BenchmarkSendQueue-4    11620237               105.7 ns/op
PASS
ok      gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/turbotunnel  3.244s
```