net/http: deflake BenchmarkClient and its use of a fixed port for testing
net/http: deflake BenchmarkClient and its use of a fixed port for testing

Let the kernel pick a port for testing, and have the server in the
child process tell the parent (benchmarking) process the port that
was selected.

Fixes flakes like seen in https://golang.org/cl/27050 (and previously)

Change-Id: Ia2b705dc4152f70e0a5725015bdae09984d09d53
Reviewed-on: https://go-review.googlesource.com/27051
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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