Added file path validity checks to file sensor (#12505)

* Added file validity checks to file sensor

* Patched out 'is_allowed_path' for file sensor tests

tests/components/sensor/test_file.py

@@ -18,6 +18,8 @@ class TestFileSensor(unittest.TestCase):
     def setup_method(self, method):
         """Set up things to be run when tests are started."""
         self.hass = get_test_home_assistant()
+        # Patch out 'is_allowed_path' as the mock files aren't allowed
+        self.hass.config.is_allowed_path = Mock(return_value=True)
         mock_registry(self.hass)
 
     def teardown_method(self, method):