New test driver to allow for standalone regression tests.

* tests/testdrv.c: New.

This driver is intended to run easily run the regression tests on the
actual host platform where the software has been compiled for. In
particular for Windows cross-building is mandatory and manually
running tests is error prone.

Some of the code has been taken from gnupg/common/exechelp*.c and
re-licensed from (LGPL-3.0-or-later OR GPL-2.0-or-later) to
LGPL-2.1-or-later. The original code has been written by g10 Code
employees or contractors.

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