math/big: fix TestFloatSetFloat64String
math/big: fix TestFloatSetFloat64String

A -0 constant is the same as 0. Use explicit negative zero
for float64 -0.0. Also, fix two test cases that were wrong.

Fixes #19673.

Change-Id: Ic09775f29d9bc2ee7814172e59c4a693441ea730
Reviewed-on: https://go-review.googlesource.com/38463
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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