Fix incorrect constants in streamlabswater tests (#119399)

tests/components/streamlabswater/__init__.py

 """Tests for the StreamLabs integration."""
 
 from homeassistant.core import HomeAssistant
-from homeassistant.util.unit_system import IMPERIAL_SYSTEM
+from homeassistant.util.unit_system import US_CUSTOMARY_SYSTEM
 
 from tests.common import MockConfigEntry
 
@@ -10,7 +10,7 @@ async def setup_integration(hass: HomeAssistant, config_entry: MockConfigEntry)
     """Fixture for setting up the component."""
     config_entry.add_to_hass(hass)
 
-    hass.config.units = IMPERIAL_SYSTEM
+    hass.config.units = US_CUSTOMARY_SYSTEM
 
     await hass.config_entries.async_setup(config_entry.entry_id)
     await hass.async_block_till_done()