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 <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>