net: fix inconsistent error values on Dial, Listen partially
This change makes TestDialError, TestListenError work without any
external dependency, enables them by default, and removes unnecessary
-run_error_test flag for fixing #4856.
Also fixes inconsistent error values on Dial, Listen partially as a
first stab.
Updates #4856.
Change-Id: Ie10c151ae06759085f352c7db2ca45107a81914f
Reviewed-on: https://go-review.googlesource.com/8903 Reviewed-by: Ian Lance Taylor <iant@golang.org>