[dev.typeparams] cmd/compile: make TestUnifiedCompare insensitive to default -G level

The test currently fails if the default -G level is changed from 0 to
3, and it only makes sense to run in -G=0 mode anyway. So might as
well hard code it that way.

Change-Id: I026d25d567157df5d3f2ca3c68d31d75d9c74532
Reviewed-on: https://go-review.googlesource.com/c/go/+/334910


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>