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 <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

vsekhar/elastic-go,Cofyc/go,nawawi/go,codestation/go,Cofyc/go,ImJasonH/go,polyverse-security/go,akutz/go,CAFxX/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,goodlang/good,nawawi/go,nawawi/go,linux-on-ibm-z/go,CAFxX/go,ImJasonH/go,derekmarcotte/go,suncycheng/go,suncycheng/go,suncycheng/go,vsekhar/elastic-go,vsekhar/elastic-go,Cofyc/go,josharian/go,vsekhar/elastic-go,golang/go,linux-on-ibm-z/go,Cofyc/go,christopher-henderson/Go,derekmarcotte/go,codestation/go,cloudflare/go,vsekhar/elastic-go,goodlang/good,josharian/go,cloudflare/go,ImJasonH/go,codestation/go,goodlang/good,polyverse-security/go,polyverse-security/go,akutz/go,Cofyc/go,codestation/go,vsekhar/elastic-go,suncycheng/go,golang/go,polyverse-security/go,cloudflare/go,derekmarcotte/go,goodlang/good,cloudflare/go,polyverse-security/go,ImJasonH/go,ImJasonH/go,golang/go,akutz/go,nawawi/go,golang/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,goodlang/good,ImJasonH/go,derekmarcotte/go,ImJasonH/go,nawawi/go,codestation/go,derekmarcotte/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,golang/go,cloudflare/go,josharian/go,Cofyc/go,derekmarcotte/go,suncycheng/go,akutz/go,CAFxX/go,nawawi/go,suncycheng/go,Cofyc/go,golang/go,linux-on-ibm-z/go,cloudflare/go,nawawi/go,josharian/go,linux-on-ibm-z/go,CAFxX/go,akutz/go,josharian/go,goodlang/good,akutz/go,linux-on-ibm-z/go,ImJasonH/go,polyverse-security/go,goodlang/good,CAFxX/go,CAFxX/go,josharian/go,derekmarcotte/go,golang/go,codestation/go,linux-on-ibm-z/go,suncycheng/go,josharian/go,cloudflare/go,cloudflare/go,polyverse-security/go,akutz/go,akutz/go,linux-on-ibm-z/go,codestation/go,Cofyc/go,golang/go,suncycheng/go,goodlang/good