tests: Support tests that are expected to fail.

* tests/gpgscm/tests.scm (test-pool): Rework reporting.  Filter using
the computed test status instead of the return value.  Also print the
new categories 'failed expectedly' and 'passed unexpectedly'.
(test): If a test ends with a bang (!), it is expected to fail.  Adapt
status, status-string, and xml accordingly.

Allow tests to be marked as being expected to fail by appending a bang
(!) to the tests name. If such a test fails, it will not be counted
as failure, but will still be prominently displayed in the report. If
it succeeds unexpectedly, this is counted as a failure.

Fixes T3134.

• GnuPG-bug-id: T3134
• Signed-off-by: Justus Winter <justus@g10code.com>