Run akonadi/kmime unit-tests only when KDEPIMLIBS_RUN_KDEPIMRUNTIME_ISOLATED_TESTS is defined

These tests depend on the akonadi_maildir_resource, which is provided by
kdepim-runtime, so they cannot be run in Jenkins, where kdepim-runtime is
not available during kdepimlibs compilation, as that would create circular
dependencies.

Also the KMime tests are quite broken (since a long time) and I don't really
understand how to fix them.