Mark failing Colab notebook tests as XFAIL. (#19031)

See #18919.

Tested here: https://github.com/iree-org/iree/actions/runs/11689771962

* I was able to reproduce the `tensorflow_hub_import.ipynb` compile
failure in Colab. Might be a real compiler regression in there.
* I was _not_ able to reproduce the `tflite_text_classification.ipynb`
error - likely some diff between the simulated test environment we have
and what is running in hosted Colab instances.

samples/colab/test_notebooks.py

@@ -22,7 +22,9 @@
 ]
 
 NOTEBOOKS_EXPECTED_TO_FAIL = [
-    # None!
+    # See https://github.com/iree-org/iree/issues/18919
+    "tensorflow_hub_import.ipynb",
+    "tflite_text_classification.ipynb",
 ]