Report skipped tests in blossom (#68)

For the tests running in blossom, generate a report of the skipped tests.

For instance on the nightly TF container this shows

```
-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================== short test summary info ================================================
SKIPPED [1] tests/unit/io/test_avro.py:34: could not import 'uavro': No module named 'uavro'
```

ci/test_unit.sh

@@ -22,5 +22,5 @@ cd /core/
 git pull origin main
 
 # Run tests
-pytest /core/tests/unit
+pytest -rxs /core/tests/unit