os: skip TestRemoveAllRace on dragonfly

os: skip TestRemoveAllRace on dragonfly

This test occasionally fails on the dragonfly-amd64 builder with
"directory not empty". Since that is the only platform on which we
observe these failures, and since the test had a different (and also
invalid-looking) failure mode prior to this one (in #50716), we
suspect that it is due to either a bug in the platform or a
platform-specific Go bug.

For #52301.

Change-Id: Id36c499651b9c48e6b8b0107d01f73d2a7b6bab8
Reviewed-on: https://go-review.googlesource.com/c/go/+/402155
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>