test/typeparam: add test for indexing on typeparams with various constraints
test/typeparam: add test for indexing on typeparams with various constraints

Add test for indexing on variables whose types are constrained to
various kinds of types.

Change-Id: I991eecfe39dba5d817c0fbe259ba558d4881ea84
Reviewed-on: https://go-review.googlesource.com/c/go/+/360867
Trust: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>
Run-TryBot: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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