Add startup time and boot_timestamp (#3136)

* Add startup time

* Add boot_timestamp

* Fix type

* patch and rename properties fixture

tests/dbus/test_systemd.py

@@ -0,0 +1,27 @@
+"""Test hostname dbus interface."""
+
+from unittest.mock import patch
+
+from supervisor.coresys import CoreSys
+from supervisor.dbus.const import DBUS_NAME_SYSTEMD
+
+from tests.common import load_json_fixture
+
+
+async def test_dbus_systemd_info(coresys: CoreSys):
+    """Test coresys dbus connection."""
+    assert coresys.dbus.systemd.boot_timestamp is None
+    assert coresys.dbus.systemd.startup_time is None
+
+    await coresys.dbus.systemd.connect()
+
+    async def mock_get_properties(dbus_obj, interface):
+        return load_json_fixture(
+            f"{DBUS_NAME_SYSTEMD.replace('.', '_')}_properties.json"
+        )
+
+    with patch("supervisor.utils.gdbus.DBus.get_properties", new=mock_get_properties):
+        await coresys.dbus.systemd.update()
+
+    assert coresys.dbus.systemd.boot_timestamp == 1632236713344227
+    assert coresys.dbus.systemd.startup_time == 45.304696