gpgscm: Guard use of tagged expressions.

* tests/gpgscm/init.scm (vm-history-print): Check that the tag added
to expressions when parsing source files matches the expected format.
* tests/gpgscm/lib.scm (assert): Likewise.

This makes the error handling more robust. We saw the assumption
about the format of the tags being violated on one build system, and
it obscured the view on the underlying problem.

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