net: TestPacketConn failures with "i/o timeout"

[bcmills]

darwin-amd64-nocgo at c515852732a490bab64f35d001ddc444b0f0f553

…
--- FAIL: TestPacketConn (40.39s)
    packetconn_test.go:79: write unixgram /var/folders/kh/5zzynz152r94t18yzstnrwx80000gn/T/workdir-host-darwin-10_15/tmp/go-nettest742938503->/var/folders/kh/5zzynz152r94t18yzstnrwx80000gn/T/workdir-host-darwin-10_15/tmp/go-nettest590542138: i/o timeout
FAIL
FAIL	net	64.287s

2021-01-08T15:41:36-c6513bc/linux-riscv64-jsing
2020-10-27T19:52:40-c515852/darwin-amd64-nocgo
2020-10-25T05:19:20-7930d39/linux-riscv64-jsing
2020-10-20T02:32:44-9ad090c/linux-riscv64-jsing
2020-09-17T12:37:40-967465d/openbsd-arm-jsing
2020-07-09T20:19:47-574dac9/openbsd-arm-jsing
2020-07-07T16:14:42-18d042e/openbsd-arm-jsing
2020-07-03T03:31:29-dd15017/openbsd-arm-jsing
2020-07-01T09:07:18-84152d5/openbsd-arm-jsing
2020-06-30T20:14:48-96e8366/openbsd-arm-jsing
2020-06-25T02:59:06-d7e3a16/openbsd-arm-jsing
2020-06-23T00:10:24-968e18e/openbsd-arm-jsing
2020-06-19T02:11:35-3dec253/openbsd-arm-jsing
2020-06-17T05:23:15-340efd3/openbsd-arm-jsing
2020-06-15T19:05:29-15e3e0d/openbsd-arm-jsing
2020-06-11T14:40:28-f7ba82d/openbsd-arm-jsing
2020-06-10T01:31:35-b9332ed/openbsd-arm-jsing
2020-06-05T19:17:58-d282b0f/openbsd-arm-jsing
2020-06-04T10:53:46-b371f18/openbsd-arm-jsing
2020-05-31T02:07:28-f1f8f9a/openbsd-arm-jsing
2020-05-31T00:55:05-8da7862/openbsd-arm-jsing
2020-05-29T20:55:14-c2d1df6/openbsd-arm-jsing
2020-05-29T09:21:54-1519bc4/openbsd-arm-jsing
2020-05-26T22:28:45-b2ce393/openbsd-arm-jsing
2020-05-20T19:21:43-daf70d6/openbsd-arm-jsing
2020-05-15T16:15:25-2b70ffe/openbsd-arm-jsing
2020-05-13T18:00:00-b819adf/openbsd-arm-jsing
2020-05-12T19:15:34-cb11c98/openbsd-arm-jsing
2020-05-10T14:43:46-57e32c4/openbsd-arm-jsing
2020-05-09T04:46:25-0242d46/openbsd-arm-jsing
2020-05-08T20:44:01-7cbee12/openbsd-arm-jsing
2020-05-08T20:28:57-26de581/openbsd-arm-jsing
2020-05-08T20:24:33-8f4be42/openbsd-arm-jsing
2020-05-08T13:29:11-b1a48af/openbsd-arm-jsing
2020-05-08T02:00:35-43f2f50/openbsd-arm-jsing
2020-05-08T00:05:04-9d1e120/openbsd-arm-jsing
2020-05-07T14:29:38-cb14bd8/openbsd-arm-jsing
2020-05-06T01:27:05-fdb8a3e/openbsd-arm-jsing
2020-05-03T07:23:32-53f2747/openbsd-arm-jsing
2020-05-01T06:37:35-fffe622/openbsd-arm-jsing
2020-05-01T05:30:49-b8fd3ca/openbsd-arm-jsing
2020-04-29T11:57:50-07d9ea6/openbsd-arm-jsing
2020-04-27T11:55:27-bce1e25/openbsd-arm-jsing
2020-04-24T22:01:49-d8d3542/darwin-amd64-race
2020-04-24T08:21:27-82f2989/openbsd-arm-jsing
2020-04-16T17:52:53-8c00e07/openbsd-arm-jsing

See previously #5524. It's not clear to me whether this is a symptom of a real bug in the net package, an unreasonably short hard-coded timeout in the test, or something else.

CC @bradfitz @ianlancetaylor @4a6f656c