net: re-enable or delete TestLookupLongTXT

[0intro]

CL 79555 added TestLookupLongTXT. This test is failing on Plan 9.

--- FAIL: TestLookupLongTXT (0.05s)
	lookup_test.go:323: LookupTXT golang.rsc.io incorrect
		have ["abcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXY"]
		want ["abcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXY" "gophers rule"]
FAIL
FAIL	net	23.605s

See https://build.golang.org/log/d83b9b31e94b0b5093c0d2813f78105531e8de3e

[0intro]

It seems the Plan 9 DNS resolver (ndb/dns) only returns a single TXT record. It probably should be fixed.

cpu% ndb/dnsquery
> rsc.io ip
rsc.io ip	216.239.38.21
rsc.io ip	216.239.34.21
rsc.io ip	216.239.32.21
rsc.io ip	216.239.36.21
> golang.rsc.io txt
golang.rsc.io txt	gophers rule
> gandi.net txt
gandi.net txt	google-site-verification: OyQ-vUKOBlZ2onEvCpyG7UGmQC1_tahPOUSNqCWYruo

[gopherbot]

Change https://golang.org/cl/79695 mentions this issue: net: skip TestLookupLongTXT on Plan 9

[bradfitz]

@0intro, feel free to send plan9 fixes whenever, but I'm removing this from Go1.10. If the fix only touches plan9 stuff, I'm comfortable taking it even late in the cycle.

[mvdan]

Different, potentially related failure: https://storage.googleapis.com/go-build-log/d38832a2/linux-amd64_b2d10b95.log

--- FAIL: TestLookupLongTXT (1.14s)
	lookup_test.go:342: LookupTXT golang.rsc.io incorrect
		have ["gophers rule"]
		want ["abcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXYabcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXY" "gophers rule"]

This happened on an unrelated CL, https://go-review.googlesource.com/c/go/+/77253 - linux-amd64 at d38832a2029f83e62a9ddfccfff6c2fc4e3a393e.