build: New configure option --enable-all-tests.

* configure.ac: New option --enable-all-tests.
* tests/gpgscm/ffi.c (ffi_init): New gloabl var *run-all-tests*.
* tests/openpgp/all-tests.scm (all-tests): Use that var instead
of *maintainer-mode*.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Add --enable-all-tests.

It is better to have a separate option to run all tests than to put
this on top of --enable-maintainer-mode. This way we can also make
sure to run all tests during "make distcheck".

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