tests: Refine the repl function.

* tests/gpgscm/repl.scm (repl): Add an argument 'environment'.
(interactive-repl): Add an optional argument 'environment'.

With this change, we can drop

(interactive-repl (current-environment))

anywhere into the code and do some interactive debugging.

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