net: Unix dnsclient test for CVE-2021-33195 assumes that 1.2.3.4 does not resolve [1.15 backport]

[cagedmantis]

@neild requested issue #46504 to be considered for backport to the next 1.15 minor release.

@gopherbot please backport to 1.15

[cagedmantis]

This change is in support of a previously submitted change. It is also just an update to a test. For those reasons, it is approved.

[gopherbot]

Change https://golang.org/cl/324333 mentions this issue: [release-branch.go1.15] net: don't rely on system hosts in TestCVE202133195

[gopherbot]

Closed by merging 6b411e9 to release-branch.go1.15.