gpgscm: Add 'finally', rework all macros.

* tests/gpgscm/init.scm (finally): New macro.
* tests/gpgscm/tests.scm (letfd): Rewrite.
(with-working-directory): Likewise.
(with-temporary-working-directory): Likewise.
(lettmp): Likewise.

Rewrite all our macros using 'define-macro'. Use the new control flow
mechanism 'finally', or 'dynamic-wind' where appropriate. Make sure
the macros are hygienic. Reduce code duplication.

• Signed-off-by: Justus Winter <justus@g10code.com>