math: add large exact float rounding tests

math: add large exact float rounding tests

This CL adds trunc,ceil,floor tests for large exact float.

Change-Id: Ib7ffec1d2d50d2ac955398a3dd0fd06d494fcf4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/601095
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>