net: enable timeout tests on Plan 9

net: enable timeout tests on Plan 9

Deadlines have been implemented on Plan 9 in CL 31521.

Enable the following tests:

 - TestReadTimeout
 - TestReadFromTimeout
 - TestWriteTimeout
 - TestWriteToTimeout
 - TestReadTimeoutFluctuation
 - TestVariousDeadlines
 - TestVariousDeadlines1Proc
 - TestVariousDeadlines4Proc
 - TestReadWriteDeadlineRace

Change-Id: I221ed61d55f7f1e4345b37af6748c04e1e91e062
Reviewed-on: https://go-review.googlesource.com/33196
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>