gpodder: Clean up package expression

We don't need to wrap twice, because we can actually pass the additional
XDG_DATA_DIRS via makeWrapperArgs.

The reason why I'm doing this within the patchPhase is because we can't
add shell variables from the current builder to makeWrapperArgs as that
content is going to end up in the wrapper verbatim.

In addition to this, gpodder was trying to search for its own directory
using the current program name, which I guess was another reason for the
double-wrap. We fix this now by setting gpodder_dir explicitly in the
main script.

Another main change is that we no longer set the "pythonX.Y-" prefix, so
the derivation name now is just "gpodder-3.9.0".

Last but not least, we enable the unit tests for gpodder in checkPhase.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @svenkeidel