tests: general fix for tests setup

tests: general fix for tests setup

Before this patch tests were installed into custom directory
  /usr/lib/dbus-tests/test-suites/dbus-tests
Originally, dbus wants its installed tests in
  /usr/lib/dbus/installed-tests/dbus
This path seems to be partially hardcoded in the configuration
files - it relies on defined datadir. As datadir is used
also for something else than tests, we don't want to change it.

This commit switches installation directory to the original,
and adds symbolic links in our custom directory.

Change-Id: Icf330ba3b5be59ee941d51b15a8f78a082297846