SystemOTA uses D-Bus for IPC. Tests rely on being able to run
dbus-daemon to create test bus and run services against it. This was
previously not tested in CI, which runs on top of a bare-bones container
image.

While the tests correctly detected that and skipped the affected suites,
this meant that a lot of existing code went untested.

Install dbus before running unit tests.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>