unicode: correctly handle negative runes

Is and isExcludingLatin did not handle negative runes when dispatching
to is16. TestNegativeRune covers this along with the existing uint32
casts in IsGraphic, etc. (For tests, I picked the smallest non-Latin-1
code point in each range.)

Updates #43254

Change-Id: I17261b91f0d2b5b5125d19219411b45c480df74f
Reviewed-on: https://go-review.googlesource.com/c/go/+/280493


Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>