testing: respect benchtime on very fast benchmarks
testing: respect benchtime on very fast benchmarks

When ns/op dropped below 1, the old code
ignored benchtime and reverted to 1s.

Change-Id: I59752cef88d8d73bfd5b085f5400ae657f78504e
Reviewed-on: https://go-review.googlesource.com/26664
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

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