test: add regress test for #53419

test: add regress test for #53419

This currently works with GOEXPERIMENT=unified. Add a regress test to
make sure it stays that way.

Updates #53419.

Change-Id: I2ea1f9039c59807fbd497d69a0420771f8d6d035
Reviewed-on: https://go-review.googlesource.com/c/go/+/413014
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>