test: add test for rounding to positive zero
test: add test for rounding to positive zero

Per the language spec clarification in https://golang.org/cl/14727.

Updates #12576
Updates #12621

Change-Id: I1e459c3c11a571bd29582761faacaa9ca3178ba6
Reviewed-on: https://go-review.googlesource.com/91895
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

josharian/go,cloudflare/go,josharian/go,akutz/go,golang/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,golang/go,josharian/go,josharian/go,akutz/go,codestation/go,Cofyc/go,CAFxX/go,Cofyc/go,cloudflare/go,josharian/go,Cofyc/go,nawawi/go,CAFxX/go,Cofyc/go,Cofyc/go,akutz/go,nawawi/go,akutz/go,codestation/go,codestation/go,cloudflare/go,codestation/go,CAFxX/go,josharian/go,Cofyc/go,nawawi/go,golang/go,golang/go,akutz/go,akutz/go,Cofyc/go,codestation/go,golang/go,CAFxX/go,Cofyc/go,cloudflare/go,akutz/go,codestation/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,codestation/go,codestation/go