math/big: fix %s verbs in Float tests error messages
math/big: fix %s verbs in Float tests error messages

Fatalf calls in two Float tests use the %s verb with Floats values,
which is not allowed and results in failure messages that look like
this:

    float_test.go:1385: i = 0, prec = 1, ToZero:
                     %!s(*big.Float=1) [0]
                /    %!s(*big.Float=1) [0]
                =    %!s(*big.Float=0.0625)
                want %!s(*big.Float=1)

Switch to %v.

Change-Id: Ifdc80bf19c91ca1b190f6551a6d0a51b42ed5919
Reviewed-on: https://go-review.googlesource.com/87199
Run-TryBot: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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