test: Make testing work on systems without or old systemd

* Introduce a macro to conditionally execute tests. This avoids
  skipping the entire test if some parts require systemd
* Skip the journal tests when no /etc/machine-id is present
* Change test-catalog to load the catalog from the source directory
  of systemd.
* /proc/PID/comm got introduced in v2.6.33 but travis is still
  using v2.6.32.
* Enable make check and make distcheck on the travis build