go/types: add test verifying corner-case behavior of iota
go/types: add test verifying corner-case behavior of iota

For #22341.

Change-Id: I7e586c58075e4c835ef97701db2f38bdf7558521
Reviewed-on: https://go-review.googlesource.com/71974
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

Cofyc/go,akutz/go,codestation/go,CAFxX/go,CAFxX/go,Cofyc/go,akutz/go,akutz/go,josharian/go,golang/go,nawawi/go,codestation/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,Cofyc/go,akutz/go,codestation/go,cloudflare/go,Cofyc/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,codestation/go,golang/go,cloudflare/go,josharian/go,Cofyc/go,golang/go,nawawi/go,Cofyc/go,codestation/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,josharian/go,josharian/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,christopher-henderson/Go,golang/go,codestation/go,codestation/go,josharian/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,codestation/go