Fix blocking I/O in mqtt tests (#121140)

tests/components/mqtt/test_discovery.py

@@ -1363,12 +1363,15 @@ EXCLUDED_MODULES = {
 
 
 async def test_missing_discover_abbreviations(
+    hass: HomeAssistant,
     mqtt_mock_entry: MqttMockHAClientGenerator,
 ) -> None:
     """Check MQTT platforms for missing abbreviations."""
     await mqtt_mock_entry()
-    missing = []
+    missing: list[str] = []
     regex = re.compile(r"(CONF_[a-zA-Z\d_]*) *= *[\'\"]([a-zA-Z\d_]*)[\'\"]")
+
+    def _add_missing():
         for fil in Path(mqtt.__file__).parent.rglob("*.py"):
             if fil.name in EXCLUDED_MODULES:
                 continue
@@ -1382,6 +1385,8 @@ async def test_missing_discover_abbreviations(
                     and match[0] not in ABBREVIATIONS_WHITE_LIST
                 )
 
+    await hass.async_add_executor_job(_add_missing)
+
     assert not missing