test/codegen: fix package name for test case

The codegen tests are currently skipped (see #48247). The test
added in CL 346050 did not compile because it was in the main
package but did not contain a main function. Changing the package
to 'codegen' fixes the issue.

Updates #48247.

Change-Id: I0a0eaca8e6a7d7b335606d2c76a204ac0c12e6d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/348392


Trust: Michael Munday <mike.munday@lowrisc.org>
Run-TryBot: Michael Munday <mike.munday@lowrisc.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>