tests: use manager_load_startable_unit_or_warn() to load units

Doing manager_load_unit() followed by UNIT_VTABLE(unit)->start(unit) would
result in an assertion failure in ->start() if the unit failed to load
properly. Something like this is okey-ish is tests, since the test units are
not expected to fail to load, but the reason for failure is clearer if we
fail immediately.