net: TestDialCancel is not compatible with new macOS ARM64 builders

[heschi]

TestDialCancel requires that traffic to 198.18.0.254 and 2001:2::254 be blackholed. The comment on slowDialTCP notes that "In some environments, the slow IPs may be explicitly unreachable". The network we're using for the new macOS ARM64 builders is one such environment. I skipped the test in https://go.dev/cl/402181, but it might be nice for someone more knowledgeable about this than me to look and decide what should be done.

[bcmills]

The comment on slowDialTCP notes that "In some environments, the slow IPs may be explicitly unreachable".

Hmm, I wonder if TestDialCancel should be installing slowDialTCP! It looks like the other tests that refer to slowDst4 and slowDst6 both do. (Those other tests are TestDialerFallbackDelay and TestDialParallel.)

@ianlancetaylor, @neild, @bradfitz: do you think it would make sense to install slowDialTCP for TestDialCancel?

[neild]

Looks to me like TestDialCancel (and any other test using the blackholed addresses) should install slowDialTCP. I'll send you a CL.

[gopherbot]

Change https://go.dev/cl/402454 mentions this issue: net: install slowDialTCP hook in TestDialCancel

[bcmills]

@gopherbot, please backport to Go 1.17 and 1.18. This test is currently failing on the release branches too.