net: TestDialerDualStackFDLeak failures on BSD builders

[bcmills]

2020-06-24T16:12:25-320e4ad/dragonfly-amd64-5_8
2019-10-21T23:36:02-a59808e/netbsd-arm-bsiegert

--- FAIL: TestDialerDualStackFDLeak (0.30s)
    dial_test.go:132: got 1; want 0
FAIL
FAIL	net	25.996s

Not exactly a useful test failure, but maybe it's enough of a lead for someone to work with.

CC @bradfitz @ianlancetaylor