cmd/compile: use typecheck.InitUniverse in unit tests
cmd/compile: use typecheck.InitUniverse in unit tests

Rather than ad hoc setting up the universe, just initialize it
properly.

Change-Id: I18484b952321f55eb3e1e48fd383068a4ee75f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/345475
Trust: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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