net: TestLookupHostCancel failures with "lookup invalid.invalid: no such host" on Darwin

[bcmills]

--- FAIL: TestLookupHostCancel (0.79s)
    lookup_test.go:947: LookupHost("invalid.invalid"): failed with unexpected error: lookup invalid.invalid: no such host
FAIL
FAIL	net	9.072s

2020-04-30T08:07:39-5c9a8c0/darwin-amd64-10_15
2020-04-30T00:33:38-df2862c/darwin-amd64-10_14
2020-04-21T14:57:43-af55060/darwin-amd64-10_15
2020-03-20T19:24:37-c505865/darwin-amd64-10_14
2019-10-28T18:06:12-91f3997/darwin-amd64-race

See previously #31950 (same failure mode on Windows), #25694 (same test but different failure mode on Darwin).

CC @mikioh @bradfitz @ianlancetaylor

[bcmills]

--- FAIL: TestLookupHostCancel (0.20s)
    lookup_test.go:947: LookupHost("invalid.invalid"): failed with unexpected error: lookup invalid.invalid: no such host
FAIL
FAIL	net	9.166s

2021-01-08T17:14:20-6598c65/darwin-amd64-10_15
2021-01-06T18:54:25-c9658be/darwin-amd64-10_14
2020-12-16T09:01:04-75e16f5/darwin-amd64-10_15
2020-12-14T17:17:59-a58be73/darwin-amd64-race
2020-11-10T15:05:17-81322b9/darwin-amd64-10_15
2020-11-05T00:21:39-c018eec/darwin-amd64-10_14
2020-10-26T18:20:05-ae585ee/darwin-amd64-10_14
2020-10-14T17:05:21-b95f0b1/darwin-amd64-10_14
2020-10-13T17:36:36-ad53103/darwin-amd64-10_14
2020-10-12T18:01:22-e0cb80e/darwin-amd64-10_14
2020-10-09T15:13:10-bb46b60/darwin-amd64-10_12
2020-09-02T10:07:50-7432bee/darwin-amd64-10_14
2020-08-24T14:38:38-85902b6/darwin-amd64-10_14
2020-08-16T17:05:44-8c39bbf/darwin-amd64-10_14
2020-08-15T05:58:24-c0dded0/darwin-amd64-race
2020-08-15T02:20:42-441b52f/darwin-amd64-10_15
2020-07-14T16:54:25-efbe47b/darwin-amd64-10_14
2020-07-07T03:43:31-6b34417/darwin-amd64-10_15
2020-06-03T14:08:41-58f017b/darwin-amd64-race

[bcmills]

2021-05-04T14:38:36-5e4f9b0/darwin-amd64-11_0
2021-04-05T17:42:13-9abedf4/darwin-amd64-11_0
2021-02-16T18:25:10-6f3da9d/darwin-amd64-10_12

[gopherbot]

Change https://golang.org/cl/369037 mentions this issue: net: don't check "invalid.invalid" lookup errors in TestLookupHostCancel

[bcmills]

This test has a fix CL out for review, and is a source of recurring failures on at least one builder. Marking as release-blocker via #11811.