Help needed to fix FilterActionEncryptTest::shouldEncrypt(PGP*) unit tests

[Friedrich W. H. Kossebau]

Hi,

when trying to fix the last remaining failing unit test in mailcommon I ran 
into unknown territory with PGP/QGpgME-related errors.
I would first need to learn about PGP (again), and ideally instead someone who 
is already expert could tackle this? Could you? :)

The test fails on an QGpgME::EncryptJob returning an error with the error 
string being
    "Unusable public key".
I assume it used to work at one point in time, and perhaps the data in the 
test environment no longer matches requirements? Perhaps even simply an EOL 
key?

So we need both someone clueful investigating the cause and then one coming up 
with a fix, ideally the same person ;)

How to reproduce the failing test:

1. Get latest mailcommon (master or release/21.08 both work here)
2. Apply manually single line fix patch from
   https://invent.kde.org/pim/mailcommon/-/merge_requests/12/diffs
3. make install
4. ctest -R mailcommon-filter-filteractionencrypttest --output-on-failure

Related resources:

Commit which added the test incl. the PGP test data:
https://invent.kde.org/pim/mailcommon/-/commit/
bd19def5bd49d6c9f4e008911d09a98cccaebdde

Failing test on CI, by example of openSUSE (mixed with other error fixed by 
abobe mentioned MR, but look out for "Unusable public key") :
https://build.kde.org/job/Applications/job/mailcommon/job/kf5-qt5%20SUSEQt5.15/106/testReport/junit/projectroot.src.filter/autotests/
mailcommon_filter_filteractionencrypttest/

Happy to help myself with testing or other simple things for assistance.

So, who feels challenged to solve this?
And enables us to get a mailcommon repo with no more failing unit tests? :)

Cheers
Friedrich