x/sys/unix: TestPselect failures due to shorter-than-expected timeout

[bcmills]

--- FAIL: TestPselect (0.00s)
    syscall_linux_test.go:244: Pselect interrupted after 126.458µs
    syscall_linux_test.go:256: Pselect: timeout should have been at least 2.5ms, got 2.471501ms
FAIL
FAIL	golang.org/x/sys/unix	0.572s

2020-10-22T20:17:47-fb209a7/linux-amd64-nocgo
2020-09-28T20:51:50-006507a/linux-ppc64-buildlet
2020-09-05T00:46:54-be1d343/linux-386
2020-01-24T20:44:21-9fbb57f/linux-s390x-ibm

See previously #35555.

CC @tklauser @ianlancetaylor @bradfitz

[gopherbot]

Change https://golang.org/cl/265020 mentions this issue: unix: add margin to expected timeout in TestPselect on linux