net: reorganize interface tests to avoid vague flakiness

This change reorganizes test cases for surveying network interfaces and
address prefixes to make sure which part of the functionality is broken.

Updates #7849.

Change-Id: If6918075802eef69a7f1ee040010b3c46f4f4b97
Reviewed-on: https://go-review.googlesource.com/22990Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>