New test Makefile target xtestsuite

* tests/Makefile.am (xtestsuite, xcheck): New targets.

This make target can be used to build tarball with all software
required for a regression tests on the target platform.

(cherry picked from commit ebc4d5670a1ada54ad907a4836eb8f6f573c2c38)

• Signed-off-by: Werner Koch <wk@gnupg.org>