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/8903Reviewed-by: Ian Lance Taylor <iant@golang.org>