os: TestRemoveAllRace failures with "directory not empty" on dragonfly-amd64

[bcmills]

#!watchflakes
post <- goos == "dragonfly" && pkg == "os" && test == "TestRemoveAllRace" && `directory not empty`

greplogs --dashboard -md -l -e 'FAIL: TestRemoveAllRace .*(?:\n .*)*directory not empty' --since=2022-01-01

2022-04-12T03:07:42-d4dbad5/dragonfly-amd64
2022-03-21T18:58:34-d8ce7ae/dragonfly-amd64

This failure mode seems to occur instead of the invalid argument failure mode observed in #50716 (attn @tuxillo).

[bcmills]

See previously #33403 (especially relevant because it was also reported on DragonFly).

[bcmills]

greplogs --dashboard -md -l -e 'FAIL: TestRemoveAllRace .*(?:\n .*)*directory not empty' --since=2022-04-13

2022-04-22T15:46:52-65d7345/dragonfly-amd64

[gopherbot]

Change https://go.dev/cl/402155 mentions this issue: os: skip TestRemoveAllRace on dragonfly

[tuxillo]

@bcmills seems like we should have a look at our unlinkat(2) implementation, right?

[bcmills]

Yep, looks like unlinkat(2) is the thing to check:
https://cs.opensource.google/go/go/+/master:src/os/removeall_at.go

[ianlancetaylor]

Moving to Backlog. Please comment if you disagree. Thanks.