cmd/go: make generate pass correct GOPACKAGE to XTest files

The existing behaviour of go generate is to pass GOPACKAGE=p
to all package files, including XTest files. This however is
incorrect as the package name for the XTest files is p_test.

Fixes #24594

Change-Id: I96b6e5777ec511cdcf1a6267a43f4d8c544c4af3
Reviewed-on: https://go-review.googlesource.com/103415


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>