test: Keep the test-suite.log around in case of a test failure

The addition of .DELETE_ON_ERROR will lead to the removal of the
test-suite.log in case of a test failure. Mark the rule as PRECIOUS
to keep that file around.
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
wip.. break the build.