cmd/go: don't be clever about mtime precision in test
cmd/go: don't be clever about mtime precision in test

This doesn't happen enough in the tests to be worth debugging.
Empirically, I expect this to add 5 seconds to the overall 'go test -short cmd/go'
on systems with precise file systems, and nothing on systems without them
(like my Mac).

Fixes #12205.

Change-Id: I0a17cb37bdedcfc0f921c5ee658737f1698c153b
Reviewed-on: https://go-review.googlesource.com/17953
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,Endika/go,cyberroadie/go,Samurais/go,CAFxX/go,mwhudson/go,vanloswang/go,roth1002/go,polyverse-security/go,ganboing/go-esx,Cofyc/go,cloudflare/go,kim-racktop/go,Samurais/go,ImJasonH/go,Cofyc/go,miolini/go,vsekhar/elastic-go,nawawi/go,mwhudson/go,theif519/go,ImJasonH/go,cloudflare/go,CAFxX/go,Samurais/go,4ad/go,4ad/go,codestation/go,golang/go,codestation/go,derekmarcotte/go,linux-on-ibm-z/go,vsekhar/elastic-go,CAFxX/go,polyverse-security/go,akutz/go,josharian/go,suncycheng/go,Samurais/go,vanloswang/go,polyverse-security/go,vanloswang/go,kim-racktop/go,cloudflare/go,theif519/go,linux-on-ibm-z/go,vsekhar/elastic-go,akutz/go,CAFxX/go,derekmarcotte/go,miolini/go,momchil-velikov/go,Samurais/go,mwhudson/go,sndnvaps/go,suncycheng/go,kim-racktop/go,ganboing/go-esx,sndnvaps/go,Cofyc/go,golang/go,kim-racktop/go,Endika/go,momchil-velikov/go,golang/go,christopher-henderson/Go,nawawi/go,roth1002/go,codestation/go,4ad/go,binarycrusader/go,polyverse-security/go,ganboing/go-esx,goodlang/good,sndnvaps/go,derekmarcotte/go,linux-on-ibm-z/go,theif519/go,kim-racktop/go,josharian/go,binarycrusader/go,momchil-velikov/go,theif519/go,vanloswang/go,codestation/go,theif519/go,kim-racktop/go,linux-on-ibm-z/go,goodlang/good,mwhudson/go,golang/go,Samurais/go,goodlang/good,Endika/go,miolini/go,cyberroadie/go,polyverse-security/go,polyverse-security/go,sndnvaps/go,ganboing/go-esx,4ad/go,suncycheng/go,momchil-velikov/go,cloudflare/go,josharian/go,akutz/go,miolini/go,suncycheng/go,4ad/go,ImJasonH/go,cloudflare/go,goodlang/good,linux-on-ibm-z/go,ganboing/go-esx,CAFxX/go,roth1002/go,roth1002/go,akutz/go,goodlang/good,CAFxX/go,miolini/go,sndnvaps/go,mwhudson/go,nawawi/go,sndnvaps/go,vanloswang/go,Endika/go,Endika/go,binarycrusader/go,mwhudson/go,suncycheng/go,miolini/go,akutz/go,theif519/go,cyberroadie/go,Endika/go,Cofyc/go,cloudflare/go,binarycrusader/go,vanloswang/go,golang/go,nawawi/go,Samurais/go,codestation/go,Cofyc/go,nawawi/go,josharian/go,derekmarcotte/go,akutz/go,CAFxX/go,vsekhar/elastic-go,ganboing/go-esx,golang/go,vsekhar/elastic-go,ganboing/go-esx,josharian/go,nawawi/go,miolini/go,golang/go,derekmarcotte/go,vsekhar/elastic-go,vsekhar/elastic-go,cyberroadie/go,codestation/go,linux-on-ibm-z/go,binarycrusader/go,momchil-velikov/go,binarycrusader/go,suncycheng/go,derekmarcotte/go,derekmarcotte/go,ImJasonH/go,mwhudson/go,goodlang/good,vsekhar/elastic-go,kim-racktop/go,codestation/go,binarycrusader/go,polyverse-security/go,Samurais/go,linux-on-ibm-z/go,kim-racktop/go,ImJasonH/go,cyberroadie/go,Cofyc/go,roth1002/go,goodlang/good,ImJasonH/go,golang/go,Cofyc/go,roth1002/go,cyberroadie/go,akutz/go,Cofyc/go,cloudflare/go,4ad/go,theif519/go,roth1002/go,binarycrusader/go,codestation/go,suncycheng/go,mwhudson/go,Endika/go,sndnvaps/go,vanloswang/go,momchil-velikov/go,josharian/go,polyverse-security/go,linux-on-ibm-z/go,momchil-velikov/go,CAFxX/go,ImJasonH/go,theif519/go,cloudflare/go,roth1002/go,miolini/go,4ad/go,momchil-velikov/go,cyberroadie/go,suncycheng/go,goodlang/good,josharian/go,vanloswang/go,cyberroadie/go,nawawi/go,ganboing/go-esx,4ad/go,Endika/go,derekmarcotte/go,sndnvaps/go,ImJasonH/go,josharian/go,akutz/go