net: simplify error messages in tests

This change simplifies unnecessarily redundant error messages in tests.
There's no need to worry any more because package APIs now return
consistent, self-descriptive error values.

Alos renames ambiguous test functions and makes use of test tables.

Change-Id: I7b61027607c4ae2a3cf605d08d58cf449fa27eb2
Reviewed-on: https://go-review.googlesource.com/9662


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>