[dev.typeparams] cmd/compile: add "toolstash -cmp"-like test of -d=unified

[dev.typeparams] cmd/compile: add "toolstash -cmp"-like test of -d=unified

This CL adds a longtest test to make sure -d=unified=1 produces output
identical to -d=unified=0.

Change-Id: I2c5d38f67dbc8fecd8332a91ba7cae22225b090c
Reviewed-on: https://go-review.googlesource.com/c/go/+/329429
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>