tests: Run the trust-pgp-4 test again.

* tests/openpgp/Makefile.am (XTESTS): Add trust-pgp-4.scm.
(EXTRA_DIST): Remove the test file from EXTRA_DIST.

Now that issue 2923 is fixed, the trust-pgp-4 test passes as
expected and we can enable it again. That should help prevent
a future regression on this issue.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>