net: enable TestDialParallel, TestDialerFallbackDelay and TestDialCancel on Plan 9
net: enable TestDialParallel, TestDialerFallbackDelay and TestDialCancel on Plan 9

TestDialParallel, TestDialerFallbackDelay and TestDialCancel
require dialTCP to support cancellation, which has been
implemented for Plan 9 in CL 22144.

Updates #11225.
Updates #11932.

Change-Id: I3b30a645ef79227dfa519cde8d46c67b72f2485c
Reviewed-on: https://go-review.googlesource.com/22203
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

codestation/go,vsekhar/elastic-go,Cofyc/go,momchil-velikov/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,christopher-henderson/Go,golang/go,4ad/go,cloudflare/go,momchil-velikov/go,CAFxX/go,golang/go,codestation/go,Cofyc/go,polyverse-security/go,goodlang/good,goodlang/good,derekmarcotte/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,theif519/go,theif519/go,cloudflare/go,suncycheng/go,linux-on-ibm-z/go,theif519/go,cloudflare/go,golang/go,polyverse-security/go,theif519/go,josharian/go,golang/go,josharian/go,suncycheng/go,vsekhar/elastic-go,derekmarcotte/go,ImJasonH/go,ImJasonH/go,codestation/go,derekmarcotte/go,suncycheng/go,goodlang/good,momchil-velikov/go,ImJasonH/go,vsekhar/elastic-go,linux-on-ibm-z/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,suncycheng/go,theif519/go,linux-on-ibm-z/go,akutz/go,theif519/go,nawawi/go,ganboing/go-esx,4ad/go,4ad/go,codestation/go,akutz/go,theif519/go,4ad/go,CAFxX/go,josharian/go,4ad/go,nawawi/go,4ad/go,linux-on-ibm-z/go,Cofyc/go,polyverse-security/go,Cofyc/go,golang/go,codestation/go,ImJasonH/go,josharian/go,cloudflare/go,cloudflare/go,momchil-velikov/go,polyverse-security/go,cloudflare/go,ImJasonH/go,derekmarcotte/go,akutz/go,akutz/go,goodlang/good,codestation/go,nawawi/go,polyverse-security/go,ImJasonH/go,Cofyc/go,ganboing/go-esx,ganboing/go-esx,josharian/go,momchil-velikov/go,ganboing/go-esx,vsekhar/elastic-go,cloudflare/go,momchil-velikov/go,akutz/go,suncycheng/go,theif519/go,akutz/go,josharian/go,linux-on-ibm-z/go,vsekhar/elastic-go,akutz/go,ganboing/go-esx,Cofyc/go,golang/go,CAFxX/go,codestation/go,ganboing/go-esx,linux-on-ibm-z/go,Cofyc/go,nawawi/go,akutz/go,linux-on-ibm-z/go,CAFxX/go,momchil-velikov/go,josharian/go,suncycheng/go,4ad/go,polyverse-security/go,golang/go,suncycheng/go,goodlang/good,Cofyc/go,4ad/go,ImJasonH/go,goodlang/good,ImJasonH/go,cloudflare/go,goodlang/good,suncycheng/go,ganboing/go-esx,polyverse-security/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,momchil-velikov/go,golang/go,vsekhar/elastic-go,ganboing/go-esx,goodlang/good,derekmarcotte/go