cmd/compile/internal/test: gofmt abiutils_test.go

Turns out that file is not formatted properly in the dev.regabi
branch.

Change-Id: I93125e65d5d3e8448c6ec1f077332c9bf7f0dd26
Reviewed-on: https://go-review.googlesource.com/c/go/+/292594


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>