[release-branch.go1.18] internal/fuzz: fix TestUnmarshalMarshal on MIPS

Previous value used in the float32 roundtrip used float32(math.NaN())-1
which caused the quiet/signal bit to flip, which seemed to break the
test on MIPS platforms. Instead switch to using float32(math.NaN())+1,
which preserves the bit and makes the test happy.

Possibly related to #37455
Fixes #51258

Change-Id: Ia85c649e89a5d02027c0ec197f0ff318aa819c19
Reviewed-on: https://go-review.googlesource.com/c/go/+/390214


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 63bd6f68)
Reviewed-on: https://go-review.googlesource.com/c/go/+/390418
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>