net: TestSendfilePipe failures with "Read did not time out"

[bcmills]

--- FAIL: TestSendfilePipe (0.05s)
    sendfile_test.go:310: Read did not time out
FAIL
FAIL	net	9.705s

2020-09-10T02:22:19-dfdc388/linux-arm64-aws
2019-10-25T18:44:06-096126d/darwin-arm64-corellium
2019-09-04T15:10:03-033299f/darwin-arm64-corellium
2019-08-28T02:29:54-22355d6/darwin-arm64-corellium
2019-08-27T17:00:43-483d6d9/darwin-arm64-corellium

It's not obvious to me whether this is a bug in the test's expectations or setup, or the result of slower-than-expected execution on these builders.

CC @mikioh @bradfitz @ianlancetaylor

[bcmills]

2021-01-07T17:49:47-6da2d3b/freebsd-arm64-dmgk
2020-12-04T22:08:54-4de4480/freebsd-arm64-dmgk
2020-10-23T19:57:55-f8376a5/linux-arm64-aws

[bcmills]

This failure mode is apparently not exclusive to arm64, so I suspect that the test is too timing-sensitive.
2021-04-14T16:18:34-ad44dfb/illumos-amd64
2021-03-30T01:17:14-a95454b/illumos-amd64