test/codegen: fix package name for test case
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 <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@lowrisc.org>
Run-TryBot: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@lowrisc.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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