Added: Unit test that provokes the mixed/periodicity failure

[kmokstad]

After the fixup commit 8b4e1a99cc3d4c8fc33b562c4950b3943351aed8 the only regression test using periodic boundary conditions and mixed dumps core. I think the logic behind the addPeriodicity() calls does not account for the existence of several bases. Here is a simple unit test that provokes the same error. This has to be re-thinked a bit, but until then that reg-test should be deactivated to avoid the fuss in the nightly builds.

[akva2]

test disabled.