Flaky test: TestLookupGmailTXT, got [globalsign-smime-dv=CD...KX8=]; want a record containing spf

[dagood]

This showed up once in linux-amd64-racecompile in https://dev.azure.com/dnceng/internal/_build/results?buildId=1078273&view=results, then when I removed racecompile, it showed up in linux-amd64-regabi in https://dev.azure.com/dnceng/internal/_build/results?buildId=1078855&view=logs&j=17f0ed56-45c3-5f4e-2883-c1105f3261d7&t=be2aae34-cdee-5b9f-80ac-cc542ed93061&l=352:

--- FAIL: TestLookupGmailTXT (0.00s)
    lookup_test.go:257: got [globalsign-smime-dv=CDYX+XFHUw2wml6/Gb8+59BsH31KzUr6c1l2BPvqKX8=]; want a record containing spf, google.com
    lookup_test.go:257: got [globalsign-smime-dv=CDYX+XFHUw2wml6/Gb8+59BsH31KzUr6c1l2BPvqKX8=]; want a record containing spf, google.com
FAIL
FAIL	net	12.192s

(In #52.)

It looks like these are network tests that request DNS info from gmail.com. In some cases these are only run during longtest but it looks like that behavior is explicitly overridden here, enabling them (not skipping them) if it's running in any builder:

go/src/net/dial_test.go

Lines 990 to 998 in bc0c82c

	// mustHaveExternalNetwork is like testenv.MustHaveExternalNetwork
	// except that it won't skip testing on non-mobile builders.
	func mustHaveExternalNetwork(t *testing.T) {
	t.Helper()
	mobile := runtime.GOOS == "android" || runtime.GOOS == "ios"
	if testenv.Builder() == "" || mobile {
	testenv.MustHaveExternalNetwork(t)
	}
	}

• https://dev.azure.com/dnceng/internal/_build/results?buildId=1100279&view=logs&j=7dcec38d-f1b9-5237-ef7c-3c012b5a4ea6&t=cabd800c-fb4c-5144-1c64-9e377443b816&l=171
• https://dev.azure.com/dnceng/internal/_build/results?buildId=1099917&view=logs&j=eaf52087-48cf-5d77-8a52-147a51debb8c&s=ff05ad62-bb9a-53b6-ce9f-72f329a63e7c&t=4872b558-a72b-5d51-a362-9995c8274f1e&l=171

---

Related issues upstream: golang/go#29698, maybe golang/go#22857, golang/go#29722