os: TestFifoEOF failures with "timed out waiting for read"

[bcmills]

2021-02-19T00:05:03-0c63312/darwin-amd64-race

--- FAIL: TestFifoEOF (1.09s)
    fifo_test.go:99: line 0
    fifo_test.go:99: line 1
    fifo_test.go:99: line 2
    fifo_test.go:107: timed out waiting for read
FAIL
FAIL	os	12.691s

2020-03-14T07:03:15-d774d97/freebsd-arm-paulzhol

--- FAIL: TestFifoEOF (1.04s)
    fifo_test.go:100: line 0
    fifo_test.go:100: line 1
    fifo_test.go:108: timed out waiting for read
    fifo_test.go:97: read /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: file already closed
    fifo_test.go:86: error closing reader: close /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: file already closed
    fifo_test.go:66: error writing to fifo: write /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: broken pipe
FAIL
FAIL	os	3.573s

Both of these failures occurred after CL 204800, which was believed to fix the problem in #35275 (CC @ianlancetaylor @mknyszek @prattmic).

See also #25877.

[bcmills]

A more thorough history:

greplogs --dashboard -md -l -e 'FAIL: TestFifoEOF .*\n(?: .*\n)* .*: timed out waiting for read'

2021-02-19T00:05:03-0c63312/darwin-amd64-race
2020-03-14T07:03:15-d774d97/freebsd-arm-paulzhol
2019-11-04T18:51:13-bf7e55b/darwin-amd64-10_14
2019-11-04T18:51:13-bf7e55b/darwin-arm64-corellium
2019-11-04T16:32:38-7dcd343/darwin-arm64-corellium
2019-11-04T15:43:40-a8fc82f/darwin-amd64-10_14
2019-11-04T10:13:30-1b2ff10/darwin-amd64-10_12
2019-11-04T10:13:30-1b2ff10/linux-amd64-nocgo
2019-11-04T05:27:25-fb29e22/darwin-amd64-10_14
2019-11-02T21:51:23-7955ece/darwin-arm64-corellium
2019-11-02T21:51:21-177a36a/darwin-arm64-corellium
2019-11-02T21:51:14-a3ffb0d/linux-386-sid
2019-11-02T21:51:12-2d031dc/darwin-amd64-nocgo
2019-11-02T19:25:54-afe50c1/darwin-amd64-10_12
2019-11-02T19:25:54-afe50c1/linux-386-longtest
2019-11-02T18:30:37-35cfe05/darwin-amd64-10_14
2019-11-02T05:52:33-8de0bb7/darwin-arm64-corellium
2019-11-02T05:52:33-8de0bb7/linux-amd64
2019-11-01T21:41:41-9bde9b4/darwin-arm64-corellium
2019-11-01T21:32:33-08a72c8/darwin-amd64-10_12
2019-11-01T20:38:31-617b416/darwin-amd64-10_12
2019-11-01T20:16:01-c9d89f6/darwin-arm-mg912baios
2019-11-01T20:16:01-c9d89f6/darwin-arm64-corellium
2019-11-01T19:01:14-2c8529c/darwin-arm64-corellium
2019-11-01T07:59:09-5b31021/linux-mips64le-mengzhuo
2019-11-01T01:51:26-efd395f/darwin-arm64-corellium
2019-11-01T00:04:51-4a09a9b/darwin-amd64-10_11
2019-10-31T23:37:57-ce49f95/darwin-amd64-nocgo
2019-10-31T21:47:51-15bff20/darwin-arm64-corellium
2019-10-31T20:45:40-d6f2f83/freebsd-386-11_2
2019-10-31T17:21:56-48c0cef/darwin-386-10_14
2019-10-31T09:49:57-1085560/darwin-amd64-10_14
2019-10-31T09:49:57-1085560/darwin-arm64-corellium
2019-10-30T21:43:57-6becb03/darwin-arm-mg912baios