ref(tests): add missing __init__.py (#69560)

Adds missing `__init__.py` to a test.
This broke snuba CI for around a solid day. Got an unhelpful error
message during test collection with a misleading hint
```
________________________________________________________________________________________________________________ ERROR collecting tests/sentry/api/endpoints/relocations/artifacts/test_index.py ________________________________________________________________________________________________________________
import file mismatch:
imported module 'test_index' has this __file__ attribute:
  /Users/johnyang/code/sentry/tests/apidocs/endpoints/teams/test_index.py
which is not the same as the test file we want to collect:
  /Users/johnyang/code/sentry/tests/sentry/api/endpoints/relocations/artifacts/test_index.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
```

Turns out there are 2 tests named `test_index.py`, both do not have
`__init__.py` in their parent directory.