flaky test: TestSmallPackets

[galargh]

I've noticed TestSmallPackets to be flaky.

It failed on ubuntu with go 1.17 here: https://github.com/libp2p/go-mplex/runs/4485473110?check_suite_focus=true but passed on other OSes and on a rerun with the same setup.

Here's the output from the failed run:

=== RUN   TestSmallPackets
CPU Bound Limit:   158347	     13074 ns/op	3682451.26 MB/s
Latency Bound Limit: 512ms
Bandwidth Bound Limit: 102400Kbps
Network Bound Limit:   122653	     35489 ns/op	1050417.25 MB/s
At MaxProc 1 102400Kbps / 256ms latency:   173366	     23267 ns/op	2265277.76 MB/s
At MaxProc 2 102400Kbps / 256ms latency:   118280	     23510 ns/op	1529489.42 MB/s
Slowdown is 31.775522%
    benchmarks_test.go:58: Slowdown from mplex was >15%: 0.317755
--- FAIL: TestSmallPackets (57.05s)