test: add test case for #51521

The test case is already working with unified IR, so add it to make
sure we don't regress while finishing unified IR's support for
dictionaries.

Updates #51521.

Change-Id: Ib7c8bf9612d30cd552e8e631fd0d487dcb177f14
Reviewed-on: https://go-review.googlesource.com/c/go/+/390356


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>