cmd/go: apply same per-package flags to test and xtest builds
If package strings has a particular set of gcflags, then the strings_test
pseudo-package built as part of the test binary should inherit the same flags.
Fixes #22831.
Change-Id: I0e896b6c0f1063454300b7323f577feffbd6650b
Reviewed-on: https://go-review.googlesource.com/81496
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>