Let ExtraPluginDirsTest succeed when headless

There is no reason to let it fail just because we're running the tests
in a setup without a graphical environment; that's not what these
tests are about.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Bump to next release cycle

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>