reflect: add back signaling NaN test
reflect: add back signaling NaN test

I was a little too agressive in CL 258957 (removing 387 support) in
removing a signaling NaN test that should probably still exist.
I should have just removed the 387 skip, not the entire test.

Change-Id: I97ca53f190eb0de00e43b96629b2cae677e7dacb
Reviewed-on: https://go-review.googlesource.com/c/go/+/348209
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go